Hi

There are two ways to avoid problems with changes to about:config:

The first is really easy - do not make any changes. Firefox works very well without anything in about:config being changed.

The second is to make a complete backup of your Firefox profile (be sure to do this after all Firefox processes have ended). You can read more about how to do this at:

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

I hope that this helps.

Are you regularly modifying prefs directly via about:config?

Firefox stores all prefs that aren't default in prefs.js in the Firefox profile folder. This includes prefs Firefox uses internally (e.g. timers) and hidden prefs that are only present when they are explicitly created either by Firefox or by you. It is not recommended to restore an older copy of prefs.js as this disrupts prefs used internally. Some prefs have additional values that aren't all exposed in the user interface or interact with other prefs. This all means that you always should be careful with modifying prefs directly as you can easily forget that you made changes, so make a note of changes you make including date/time and Firefox version. Some prefs only work for a few versions and are no longer supported in newer versions, but they stay in prefs.js (about:config) since they have been modified although they no longer work. Such so longer supported prefs only have a trashcan and not an edit button on about:config just like hidden prefs that do not appear by default.