While it is easy to make customizations via Firefox’s about:config manager, it can also be easy to really mess things up. Hence the reason Mozilla has the below warning (which replaced Be Careful,This gun is loaded! warning starting with Firefox 3) the first time you enter about:config manager.

You can always change a preference back to the default by right-clicking on the preference name and selecting Reset from the context menu. This is useful if a change has borked (technical term) all or part of Firefox. But what if you have made several changes and are unsure or do not recall what you changed? At this point you may be thinking you are going to have to scrap your current profile and go through the process of starting with a fresh profile. Good news, you don’t need to go to this extreme. Firefox’s Safe Mode allows you to Reset all user preferences to Firefox defaults.

So, how do you get into safe mode? If Firefox is running, simply go to Help > Restart with Add-ons Disabled… (why it is not labeled as Restart in Safe Mode is beyond me. That seems as about as intuitive as clicking Start in Windows XP to shut down your computer). Now, if you have managed to borked your Firefox so bad it crashes upon startup, you should still be able to access safe mode by holding down the SHIFT key while Firefox is loading (I’ve never tried this and not sure it would work as I launch Firefox through the ProfileManager). Once you are entering safe mode you should see a dialog box similar to this:

To restore your about:config settings you will check the box for Reset all user preferences to Firefox defaults and then click the Make Changes and Restart button. Warning: once you do this, it can not be undone! Tip: If Firefox will still startup and you can still access about:config manager then make a backup of your prefs.js file (this is the file where all the about:config settings are stored). This could come in handy later if you remember or find what you changed that caused Firefox to stop working correctly. This way you will be able to get back your previous customizations. To backup your prefs.js follow these directions:

1. Go to Help > Troubleshooting Information
2. In the new tab under the Application Basics table click on the button Open Containing Folder next to Profile Directory
3. Close Firefox
4. In the explorer or terminal window locate the file pref.js and rename it something else (prefs.bad or prefs.old or prefs.bak).
5. Start Firefox. Since the pref.js file is not present, Firefox will automatically rebuilt it with the default settings.

If later on you do remember what you did or find the site again that gave you the instruction on what about:config settings to change, you can simply revert back to your old prefs.js file:

1. Go to Help > Troubleshooting Information
2. In the new tab under the Application Basics table click on the button Open Containing Folder next to Profile Directory
3. Close Firefox
4. In the explorer or terminal window locate the file pref.js and delete or rename the file
5. Find the your old pref.js file you had renamed in step 4 above. Rename this file to pref.js
6. Start Firefox. Now you should have all your old customizations back, including those that caused Firefox to become borked.

From here you can get into about:config and see if undoing the changes fixes Firefox. If it does not, you can always revert back to a prior version of your prefs.js file. Even if you can not get Firefox to work like it use to, this process is much quicker and less painful than starting over with a fresh profile.