You might want to "Refresh" your Firefox: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-a... It help for me. Especially if you many add-ons over time that might have done all kind of things to your settings...
Backup/Restore your bookmarks etc. afterwards as it cleans up everything. Resetting everything afterwards took 1h for me but was totally worth it in retrospective.