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 It does remove some things (often things that can cause problems, like badly out-of-date-unmaintained extensions, clears DOM storage, download history, plugin settings): https://support.mozilla.org/en-US/kb/reset-firefox-easily-fi...