Try Help -> Troubleshooting -> Reset Profile (upper right).
That fixes most problems of that sort with (often ancient, migrated since FF 2.0 or 3.0) profiles.
Most things are retained, even cookies. It does remove some things (often things that can cause problems, like badly out-of-date-unmaintained extensions, clears DOM storage, download history, plugin settings): https://support.mozilla.org/en-US/kb/reset-firefox-easily-fi...
I just spent an hour trying to figure out why ff freezes on the partner's computer. Yay 780mb sqlite wal. Who the fuck knows what that's doing. Why on earth does using a browser require vaccuming sqlite files? Dunno, because there's no good reason.
If she'll finally switch to chrome I'll stop hearing complaints the internet is slow.