This drove me crazy. The apparent solution is to delete the "parent lock" file in your Firefox profile folder. Make sure Firefox is off, go to start menu search box and type or paste %APPDATA%\Mozilla\Firefox\Profiles. Click on the file that appears in search results. This opens your profile folder. Scroll down to parent lock and delete it. You can only do this when Firefox is closed.
The other thing that helped was disabling add ons, particularly Ad Blocker and Auto Fill. This is a pain, though, disabling and enabling add ons all the time.
Firefox seems to be great when you first start using it, but over time, it really gets messed up. I seem to have fixed the problem for now, but if it starts up again, I'm defecting to Google Chrome. Too bad, because I'm used to Firefox, and Chrome isn't so user friendly, IMO.