Did you have an add-on that provided the "Black navigation bar"? If so, maybe it is not compatible with Firefox 20 which is still in beta. If this is the case it might be best to contact the add-on author to report the problem.
Clear the cache and the cookies from sites that cause problems.
"Clear the Cache":
"Remove Cookies" from sites causing problems:
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
I guess the 1st part worked because there were nothing new in extension or theme, i usually keep things at their default mode.