THREAT REMOVAL

The article will help you understand what is the Prefs.js file and how to remove it if needed. Follow the removal instructions given at the end if you are sure you need to remove it.

Prefs.js is a completely legitimate file, which is responsible for keeping the preferences for the Mozilla Firefox browser. Those preferences include Privacy settings, along with what theme is in use and for other features to stay turned on or off. As much as this file is important for the browser to run the way you like it, it is also detected by some anti-malware software programs. The reason for those detections is because malware such as a browser hijacker or adware has modified or affected that Prefs.js file and the security tool makes an association between the file and the malware. The most noticeable change you can see when Prefs.js is altered is when you browse you will be redirected or see lots of ads.

Threat Summary

NamePrefs.js
TypeJavaScript File
Short DescriptionPrefs.js is a legitimate file used by the Mozilla Firefox and browsers based on it. Sometimes AV vendors could detect that file because of scripts, false positives or mainly because the file has been affected by malware.
SymptomsIn case the file is modified by malware such as browser hijackers, redirects or adware, some browser settings get changed. The home page, new tab, extensions, themes, privacy settings could be altered. You could experience redirects, lots of new tabs popping up or multiple advertisements showing when you browse and you can’t reverse those settings no matter how you try to save them.
Distribution MethodsFreeware Installations, Bundled Packages, Extensions, Malware
Detection Tool See If Your System Has Been Affected by malware

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User ExperienceJoin Our Forum to Discuss Prefs.js.

Prefs.js – What Is It?

Prefs.js is a file that comes with the browser Mozilla Firefox and all its variations and forks, such as Nightly or PaleMoon for instance. That JavaScript file keeps various and mostly all preferences associated with Firefox browsers.

You can preview the contents of the file from the below screenshot:

Look at what is explicitly said in the beginning of that file by the developers of Mozilla Firefox:

# Mozilla User Preferences

/* Do not edit this file.
*
* If you make changes to this file while the application is running,
* the changes will be overwritten when the application exits.

This file is not made to be edited from people who do not know how to do so and what the result will be. You shouldn’t be tampering with it, but probably an application you have installed without realizing has altered the file. That can be easily spotted if you witness changes in preferences and tweaks of your browser.

The following settings and preferences are contained in Prefs.js:

  • Themes
  • Home page
  • Download location for files
  • Toggle for remembering passwords
  • History settings (as is it “saved” or not)
  • Incognito mode settings
  • Other tracking settings
  • Sending Reports and Telemetry setting
  • Update settings

As you can see above, that is quite the long list and it will take you a while to change those settings back if needed. Installing an extension could modify those preferences to something you don’t want.

Prefs.js – Where Is It Located?

The Prefs.js is located on your computer on one of your disk drives. By knowing its location you can open it with notepad, make changes, try to restore previous versions, or switch it with a new Prefs.js file to make sure it is clean.

The following Windows command is used to access a folder which contains another, randomly named folder to finally reach the Prefs.js file:

→%APPDATA%\Mozilla\Firefox\Profiles\

That command will open a folder, which has only one sub-folder, containing lots of files related to the browser, including the Prefs.js file itself. You can see the contents of that sub-folder right down here in the picture:

The full directory in this case is:

C:\Users\{name}\AppData\Roaming\Mozilla\Firefox\Profiles\nbeyyg3m.default\Prefs.js

Another way to access the file, or to make changes inside it rather, is by typing about:config into your browser. However, that will show the following warning page, before unlocking the page with the actual code:

The text states the following:

This might void your warranty!
Changing these advanced settings can be harmful to the stability, security, and performance of this application. You should only continue if you are sure of what you are doing.

Also, if you go into the directory and open the file, there will be another warning inside, which we mentioned above when describing the contents of Prefs.js. Make sure you know what you are doing, before making any changes or make a copy of the file beforehand so you have a backup if you mess things up and want to restore it.

Prefs.js – Why Is It Detected by Security Tools?

Prefs.js is detected by Anti-malware and security tools only in the case that the file is somehow connected to malware. That can happen in various ways, but it is certainly an indication of the file being tampered with or directly affected by some form of malware. In most cases that malware is a browser hijacker, redirect or adware.

The adware spams you with advertisements, which can cover the whole page; they can be triggered upon clicking on specific locations on a page, including links; or appear as pop-ups and on the side of pages on a website. The redirect is simply made to modify the browser so redirects you when a certain action is met, or from every single link and tab. The browser hijacker can be a combination of adware with redirects or something in a milder form. In all cases, that is not only highly annoying, but could end up having a serious impact on your browsing and work.

If you have any doubts or see any of the abovementioned symptoms you should immediately check your browsers and computer systems for any suspicious software and also do a scan with a security tool.

Should You Remove Prefs.js?

There is no real harm in manually locating the file and straight out deleting it. Your browser, be it the original Firefox, one of its other official editions or a fork written on the same code, will still save your Bookmarks and Add-ons. Besides, any Firefox browser will create a new Prefs.js on its launch and any new changes will be saved on it, afterward.

Here are browsers which use the file:

  • Mozilla Firefox
  • Nightly
  • Aurora
  • PaleMoon
  • Waterfox
  • Comodo IceDragon
  • xB Browser
  • Flock

All of the browser builds elicited above will have a Prefs.js, while some may put it inside the main drive they are installed on. You should just type the name of the file and do a search if it’s not found in its usual place.

In case you do not want to lose any of the preferences saved by that file or there is seemingly some malware on your system that keeps modifying the file, you should consider scanning your system by following the step-by-step removal instructions provided below. If the manual removal does not get rid of the malware causing the changes and its files completely, you should search for and remove any leftovers with an advanced anti-malware tool. Such a tool can keep your computer safe in the future.

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Berta Bilbao

Berta is a dedicated malware researcher, dreaming for a more secure cyber space. Her fascination with IT security began a few years ago when a malware locked her out of her own computer.

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Windows Mac OS X Google Chrome Mozilla Firefox Microsoft Edge Safari Internet Explorer

How to Remove Prefs.js from Windows.


Step 1: Boot Your PC In Safe Mode to isolate and remove Prefs.js

OFFER

Manual Removal Usually Takes Time and You Risk Damaging Your Files If Not Careful!
We Recommend To Scan Your PC with SpyHunter

Keep in mind, that SpyHunter’s scanner is only for malware detection. If SpyHunter detects malware on your PC, you will need to purchase SpyHunter's malware removal tool to remove the malware threats. Read our SpyHunter 5 review. Click on the corresponding links to check SpyHunter's EULA, Privacy Policy and Threat Assessment Criteria

1. Hold Windows key() + R


2. The "Run" Window will appear. In it, type "msconfig" and click OK.


3. Go to the "Boot" tab. There select "Safe Boot" and then click "Apply" and "OK".
Tip: Make sure to reverse those changes by unticking Safe Boot after that, because your system will always boot in Safe Boot from now on.


4. When prompted, click on "Restart" to go into Safe Mode.


5. You can recognise Safe Mode by the words written on the corners of your screen.


Step 2: Uninstall Prefs.js and related software from Windows

Here is a method in few easy steps that should be able to uninstall most programs. No matter if you are using Windows 10, 8, 7, Vista or XP, those steps will get the job done. Dragging the program or its folder to the recycle bin can be a very bad decision. If you do that, bits and pieces of the program are left behind, and that can lead to unstable work of your PC, errors with the file type associations and other unpleasant activities. The proper way to get a program off your computer is to Uninstall it. To do that:


1. Hold the Windows Logo Button and "R" on your keyboard. A Pop-up window will appear.


2. In the field type in "appwiz.cpl" and press ENTER.


3. This will open a window with all the programs installed on the PC. Select the program that you want to remove, and press "Uninstall"
Follow the instructions above and you will successfully uninstall most programs.


Step 3: Clean any registries, created by Prefs.js on your computer.

The usually targeted registries of Windows machines are the following:

  • HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
  • HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
  • HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnce
  • HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\RunOnce

You can access them by opening the Windows registry editor and deleting any values, created by Prefs.js there. This can happen by following the steps underneath:

1. Open the Run Window again, type "regedit" and click OK.


2. When you open it, you can freely navigate to the Run and RunOnce keys, whose locations are shown above.


3. You can remove the value of the virus by right-clicking on it and removing it.
Tip: To find a virus-created value, you can right-click on it and click "Modify" to see which file it is set to run. If this is the virus file location, remove the value.

IMPORTANT!
Before starting "Step 4", please boot back into Normal mode, in case you are currently in Safe Mode.
This will enable you to install and use SpyHunter 5 successfully.

Step 4: Scan for Prefs.js with SpyHunter Anti-Malware Tool

1. Click on the "Download" button to proceed to SpyHunter's download page.


It is recommended to run a scan before purchasing the full version of the software to make sure that the current version of the malware can be detected by SpyHunter. Click on the corresponding links to check SpyHunter's EULA, Privacy Policy and Threat Assessment Criteria.


2. After you have installed SpyHunter, wait for it to update automatically.

SpyHunter5-update-2018


3. After the update process has finished, click on the 'Malware/PC Scan' tab. A new window will appear. Click on 'Start Scan'.

SpyHunter5-Free-Scan-2018


4. After SpyHunter has finished scanning your PC for any files of the associated threat and found them, you can try to get them removed automatically and permanently by clicking on the 'Next' button.

SpyHunter-5-Free-Scan-Next-2018

If any threats have been removed, it is highly recommended to restart your PC.


Windows Mac OS X Google Chrome Mozilla Firefox Microsoft Edge Safari Internet Explorer

Get rid of Prefs.js from Mac OS X.


Step 1: Uninstall Prefs.js and remove related files and objects

OFFER
Manual Removal Usually Takes Time and You Risk Damaging Your Files If Not Careful!
We Recommend To Scan Your Mac with SpyHunter for Mac
Keep in mind, that SpyHunter for Mac needs to purchased to remove the malware threats. Click on the corresponding links to check SpyHunter’s EULA and Privacy Policy


1. Hit the ⇧+⌘+U keys to open Utilities. Another way is to click on “Go” and then click “Utilities”, like the image below shows:


2. Find Activity Monitor and double-click it:


3. In the Activity Monitor look for any suspicious processes, belonging or related to Prefs.js:

Tip: To quit a process completely, choose the “Force Quit” option.


4. Click on the "Go" button again, but this time select Applications. Another way is with the ⇧+⌘+A buttons.


5. In the Applications menu, look for any suspicious app or an app with a name, similar or identical to Prefs.js. If you find it, right-click on the app and select “Move to Trash”.


6: Select Accounts, after which click on the Login Items preference. Your Mac will then show you a list of items that start automatically when you log in. Look for any suspicious apps identical or similar to Prefs.js. Check the app you want to stop from running automatically and then select on the Minus (“-“) icon to hide it.


7: Remove any left-over files that might be related to this threat manually by following the sub-steps below:

  • Go to Finder.
  • In the search bar type the name of the app that you want to remove.
  • Above the search bar change the two drop down menus to “System Files” and “Are Included” so that you can see all of the files associated with the application you want to remove. Bear in mind that some of the files may not be related to the app so be very careful which files you delete.
  • If all of the files are related, hold the ⌘+A buttons to select them and then drive them to “Trash”.

In case you cannot remove Prefs.js via Step 1 above:

In case you cannot find the virus files and objects in your Applications or other places we have shown above, you can manually look for them in the Libraries of your Mac. But before doing this, please read the disclaimer below:

Disclaimer! If you are about to tamper with Library files on Mac, be sure to know the name of the virus file, because if you delete the wrong file, it may cause irreversible damage to your MacOS. Continue on your own responsibility!

1: Click on "Go" and Then "Go to Folder" as shown underneath:

2: Type in "/Library/LauchAgents/" and click Ok:

3: Delete all of the virus files that have similar or the same name as Prefs.js. If you believe there is no such file, do not delete anything.

You can repeat the same procedure with the following other Library directories:

→ ~/Library/LaunchAgents
/Library/LaunchDaemons

Tip: ~ is there on purpose, because it leads to more LaunchAgents.


Step 2: Scan for and remove malware from your Mac

When you are facing problems on your Mac as a result of unwanted scripts, programs and malware, the recommended way of eliminating the threat is by using an anti-malware program. Combo Cleaner offers advanced security features along with other modules that will improve your Mac’s security and protect it in the future.



Windows Mac OS X Google Chrome Mozilla Firefox Microsoft Edge Safari Internet Explorer


Remove Prefs.js from Google Chrome.


Step 1: Start Google Chrome and open the drop menu


Step 2: Move the cursor over "Tools" and then from the extended menu choose "Extensions"


Step 3: From the opened "Extensions" menu locate the unwanted extension and click on its "Remove" button.


Step 4: After the extension is removed, restart Google Chrome by closing it from the red "X" button at the top right corner and start it again.


Windows Mac OS X Google Chrome Mozilla Firefox Microsoft Edge Safari Internet Explorer


Erase Prefs.js from Mozilla Firefox.

Step 1: Start Mozilla Firefox. Open the menu window


Step 2: Select the "Add-ons" icon from the menu.


Step 3: Select the unwanted extension and click "Remove"


Step 4: After the extension is removed, restart Mozilla Firefox by closing it from the red "X" button at the top right corner and start it again.



Windows Mac OS X Google Chrome Mozilla Firefox Microsoft Edge Safari Internet Explorer


Uninstall Prefs.js from Microsoft Edge.


Step 1: Start Edge browser.


Step 2: Open the drop menu by clicking on the icon at the top right corner.


Step 3: From the drop menu select "Extensions".


Step 4: Choose the suspected malicious extension you want to remove and then click on the gear icon.


Step 5: Remove the malicious extension by scrolling down and then clicking on Uninstall.



Windows Mac OS X Google Chrome Mozilla Firefox Microsoft Edge Safari Internet Explorer


Remove Prefs.js from Safari.


Step 1: Start the Safari app.


Step 2: After hovering your mouse cursor to the top of the screen, click on the Safari text to open its drop down menu.


Step 3: From the menu, click on "Preferences".

stf-safari preferences


Step 4: After that, select the 'Extensions' Tab.

stf-safari-extensions


Step 5: Click once on the extension you want to remove.


Step 6: Click 'Uninstall'.

stf-safari uninstall

A pop-up window will appear asking for confirmation to uninstall the extension. Select 'Uninstall' again, and the Prefs.js will be removed.


How to Reset Safari
IMPORTANT: Before resetting Safari make sure you back up all your saved passwords within the browser in case you forget them.

Start Safari and then click on the gear leaver icon.

Click the Reset Safari button and you will reset the browser.


Windows Mac OS X Google Chrome Mozilla Firefox Microsoft Edge Safari Internet Explorer


Eliminate Prefs.js from Internet Explorer.


Step 1: Start Internet Explorer.


Step 2: Click on the gear icon labeled 'Tools' to open the drop menu and select 'Manage Add-ons'


Step 3: In the 'Manage Add-ons' window.


Step 4: Select the extension you want to remove and then click 'Disable'. A pop-up window will appear to inform you that you are about to disable the selected extension, and some more add-ons might be disabled as well. Leave all the boxes checked, and click 'Disable'.


Step 5: After the unwanted extension has been removed, restart Internet Explorer by closing it from the red 'X' button located at the top right corner and start it again.


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