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I would like to revert Firefox to a previous version and keep it that way.

I understand that the developers don't want users doing this.

I have tried simply uninstalling and re-installing the proper version (their instructions here), but when I try to start Firefox afterwards, it prevents me from importing my profile. Firefox does this if the profile is newer than the installed version. In this question, I am asking how to revert and maintain my profile.

I have searched Superuser but don't see an answer. This answer is about old profiles; my question is actually about profiles newer than the installed version. This answer is also about copying an older profile. This answer is about going from 32-bit to 64.

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2 Answers 2

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These are instructions to install the version of Firefox you want and keep it that version, while also keeping your user profile intact. They are for Windows, although the real key to starting Firefox after the change (adding --allow-downgrade to your startup shortcut) is the same for Linux.

  1. Make a backup of your Profiles folder. This step can be skipped, although aside from protecting data, it's helpful to be able to refer to the original directory structure later.

    On Windows your profile folder is in %APPDATA%\Mozilla\Firefox\Profiles\. In recent versions it will have a name like "nq6fgrkl8.default", but yours could be named something different.

    You can also locate this folder from Firefox by going to Hamburger menu->Help->Troubleshooting Information->Application Basics->Profile Folder->Open Folder. Since Firefox cannot be running while we update it, this must be done before anything else.

  2. Download the version of Firefox you want. The previous version is on this page. A directory of all previous versions is here.

  3. Uninstall Firefox. There may be associated pieces of software with it, also called "Mozilla Firefox [name]", like the updater. Uninstall those too.

  4. Reboot.

    (The first time I tried this I did not reboot, and it turned out Firefox 88 was actually still installed. The About menu in Firefox said version 86, but it was 88, and the Windows list of installed programs showed 88. Possibly you can just uninstall Firefox properly and not have to reboot like I did, but browsers get their hooks deep into everything, and in Windows especially, a reboot is best.)

  5. Install your chosen Firefox version. Don't start it yet. If you accidentally start Firefox, you may need to reboot again, or even restart the process.

  6. Locate your new profile directory (directions in step 1). It will be in the same directory as the old one and also have an unpronouncable name. Copy the contents of your old directory to the new one, overwriting anything there.

    Firefox cannot be running when you do this because it locks some of the profile files while running, preventing a clean copy.

  7. Now Firefox has been reinstalled and your settings copied, but when it detects that your profile is from an older version, it will refuse to start.

    Edit your Firefox startup shortcut. You need to add the flag --allow downgrade at the end. This also works for Linux.

  8. Now Firefox will start, but it will immediately try to update itself, and it will occasionally notify you about it. There are two ways to stop the updates:

    A. Stop updates and the notification using the policies.json technique by Brethlosze:

    • Create the folder distribution at the same place the Firefox executable is located (i.e. in Windows: C:\Program Files\Mozilla Firefox\distribution\).
    • Create inside, a file policies.json with this text:
    { 
       "policies": {
           "DisableAppUpdate": true
       } 
    }
    

    B. Stop the updates from within Firefox. Start Firefox using your new shortcut. Before doing anything else, go to about:preferences -> Firefox Updates, and turn off automatic updates.

    With this method Firefox will popup a notification on startup, and occasionally during use, asking you to update. If you accidentally click it it will attempt to install, and you may have to repeat all of this if it does. (If you do accidentally click it you may be able to stop installation by declining the Windows popup that confirms installation, however I have not tested this.)

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I used this to downgrade to the latest ESR version after trying to pin down yet another font rendering bug in the last Release channel version, but it should work for any Firefox edition. If you do want to downgrade, I recommend the ESR edition - because it's designed for enterprise customers that need stability and predictability (as opposed to us plebs that obviously don't), it receives no shiny feature updates (and therefore no new bugs) while still receiving regular security updates.


  1. Download your desired version of Firefox and install it - it's not necessary to uninstall the old version as the installer is capable of installing in-place (to the same directory) without touching your data.

  2. Once installed, try opening Firefox. If by some miracle you don't get the dreaded "warning" that won't let you go any further because your user profile is too new, you're good to go and you can skip the rest of these steps. If you do get this warning, click Exit.

  3. Open %USERPROFILE%\AppData\Roaming\Mozilla\Firefox\Profiles (copy and paste into the Explorer address bar if using Windows) and click on the folder containing your user profile - this should be the only one that has your name at the end of it.

  4. Look for a file called compatibility.ini - delete it, then restart your PC. When you next open Firefox, you should now be taken through to your user profile as normal.

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