I uninstalled Firefox and deleted everything labeled “Firefox”, and got help here in deleting all the old settings. I used Adobe’s uninstall tool for Flash, then reinstalled it. Ff still hates Flash, and Flash immediately crashes when I go to pages with it embedded. I can’t play Facebook or Youtube videos in Ff.
I had similar issues. I Uninstalled and Deleted the folder. Reinstalled. Flash sites still wouldn’t load correctly. Then I found the reset and it fixed it.
I’m not sure where Firefox stores those settings. I’d guess somewhere in the user profile. uninstalling and even deleting the FF folder won’t get rid of it.