[signum]

OK, next step. Start Firefox from the command line in "safe mode" -

firefox -safe-mode

and see if the black bar still appears. This is not something you want to use all the time, it's just to narrow down the problem to see if it's fixable in a certain way. The safe mode will disable all extensions, themes, file associations, user customizations, etc., basically resetting it to factory defaults. But only temporarily - i.e., for the duration of the session. Nothing is changed on the disk.

(edit) When you start Firefox in safe mode, you'll get a popup window with a choice of "Reset Firefox" or "Start in Safe Mode" buttons. Click on "Start in Safe Mode" at this time. "Reset" is a serious commitment.