One reason for this problem to occur would be some corruption in Firefox's database of extensions. You can force Firefox to regenerate the files by doing the following:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename any of these files that you find:

• compatibility.ini to something like compatibility-old.ini
• extensions.ini to something like extensions-old.ini
• extensions.json to something like extensions-old.json
• extensions.sqlite to something like extensions-old.sqlite (might not exist depending on how long you've been using this settings folder)

When you start Firefox back up again, it should explore your profile folder and other folders to rebuild the list. It probably will ask you to approve some of the extensions. Once things settle down, could you check the Add-ons page again.

Any improvement?

If the above hasn't helped then also delete the addons.json file.

You can also check for problems with preferences.

Delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

jscher2000 - your 2nd solution worked perfectly! Thanks for solving the problem, and for the very clear and detailed instructions both times.

For your own reference, i didn't have a full theme installed, just a themed toolbar. I had changed nothing at all within Firefox or on my pc when the problem occurred. It was purely updating to 42.0 that caused it.

Once again, many thanks! :)

Seems i spoke to soon about this being solved yesterday. The first reply did initially solve the problem, and after closing and restarting Firefox several times everything was still ok, but i stupidly didn't try completely shutting down and restarting the pc at all. I've started it up tonight for the first time since the fix and it's reverted to the original state i first described.

I haven't tried the second solution yet because as far as i can tell (though please correct me if i'm wrong) that's for if the preferences are lost as soon as Firefox is closed, which is not happening - they're only lost when the pc is shut down. Also if i understand it correctly this will reset Firefox completely and lose everything i've personalised or added? - I don't particularly want to do that at this stage.

I've also tried removing the add-ons completely, carrying out the 1st solution, then re-installing the add-ons from scratch, but the problem is the same still. Though as i said before they are still installed and running despite not being shown in the add-ons window.

Any help would be appreciated - thanks!

Hmm, a user in another thread indicated extensions disappearing at Windows startup. Sometimes "cleaning" software will modify files in Firefox's profile folder on a schedule, including at Windows startup. If you have any software like that, can you exclude Firefox?

Two other thoughts:

(1) Do you use a full theme? That's more than just a pretty toolbar background and can affect the functioning of built-in pages.

(2) In case this is a problem with damaged program files, you can try this:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 42.0 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename just the current program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in (A). It should automatically connect to your existing settings.

Does the Add-ons page work in the reinstalled Firefox? If not, there's still a problem in the profile folder.

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Otherwise, there's no reason to keep the old folder except research/curiosity.

If some cleanup or security software is causing this problem then you need to fix this in that software. You can't do anything about this in Firefox when this software restore a 'trusted' state of some files in the profile folder on every Windows boot.

You can backup the entire profile folder or at least the extensions.* and prefs.js files immediately after closing Firefox and check if you still have the same files after a reboot.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10