as last resort you can reset FF but you will lose settings and bookmarks unless you back it up .. Help --> troubleshooting Info --> reset FF
I did with a back up what you said but nothing changed. Lastly I uninstalled firefox and downloaded a latest version,tried to install it with caution from eset but saw same thing. At last I uninstalled both eset and firefox again reinstalled them , now I think I'm fine.
several free alternatives, that are close in par to many paid ones (some may say better than paid ones). It's all up to the individual and their needs imo.