To tweak Firefox UI or to disable a feature if you’ve changed a preference value in about:config, that’s fine! You can always reset the same preference value to default easily. That’s not the case, if you’ve made changes to multiple preferences that are hidden and you don’t remember or not noted those changed preferences, then reinstalling or refreshing Firefox are things you can do, but without needing to do either of those, you can reset Firefox’s all about:config preferences.
Resetting All of Firefox’s about:config preferences to default
1. Open Firefox browser, visit about:support page
2. Click on ‘Open Folder’ and close the Firefox browser.
3. Backup prefs.js file — which contains all about:config preference settings — to a separate location and delete it from the default profile folder.
4. Restart Firefox browser, new prefs.js file will be automatically generated with default values for about:config entries when you launch Firefox browser next time, done.
Note: Doing the above also removes the preferences left by extensions after their uninstall.