By default, this preference should not be locked, and set to true as well.

Do all names in the line for this preference display in bold, and is it set to false, compared to other preferences?

Some theories: - Perhaps your Windows system is not set to double-click globally. -> Can you right-click the preference and choose "Toggle" instead? - You may have an add-on responsible for "locking" the preference. -> Can you double-click and modify the preference when you temporarily start Firefox in its Safe Mode, which disables all add-ons? - An add-on or other piece of software created a user.js file in your profile folder. -> Have a look at this support article to read how you can find and (re)move the file.

Also see; https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

If this extension has status locked then it is likely locked via a mozilla.cfg file in the Firefox program folder. You can click the status header to sort by this column to see if more prefs are locked.

Check if there is a mozilla.cfg file in the Firefox program folder to set or lock preferences.

This requires a local-settings.js file in the "defaults/pref" folder where the channel-prefs.js file is located that specifies to use mozilla.cfg.

If there are .js files present in the "defaults\pref" folder apart from the channel-prefs.js file that sets app.update.channel then open this file in a plain texteditor like Notepad (Mac: TextEdit) and please post its content. Also please post the content of the mozilla.cfg file.

There shouldn't be a mozilla.cfg file present unless you've created it yourself to initialize some prefs.

If you have this mozilla.cfg file then best is to do a clean reinstall and delete the Firefox program folder to make sure that all files are removed before reinstalling a fresh copy of Firefox.

• http://kb.mozillazine.org/Preferences_not_saved#Other_solutions

See also Configuration:

• https://developer.mozilla.org/en-US/Firefox/Enterprise_deployment