Issues with a black band or black (empty) toolbar can be caused by a plugin or an extension that is not compatible with Firefox 3.6 and isn't working properly anymore.
A toolbar like the "ZoneAlarm Spy Blocker Plugin Stub for 32-bit Windows" and other toolbars like "Ask.com" or "My Web Search" can cause this issue.

See http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

Thanks cor-el .... will check all the plug-ins and not just the add-ons.

Ok this gets weird! After disabling each and every add-on, I found just one that impacts on the appearance of the cursed black horizontal stripe. It's the Zone Alarm Spy-Blocker.

But disabling that stub does NOT get rid of the stripe. It merely changes the stripe's appearance to diagonal shades of grey with the plug-in icon in the middle.

So, where now please?

) Problem solved so simply and quickly.... After despairing of a solution I idly googled the problem and came upon this site: http://www.dslreports.com/forum/r23684455-Extension-Persona-messed-up-my-Firefox~start=20

..turns out I'm not the only FF user with the problem.

The solution is so simple it beggars belief> Go to View, Toolbars and uncheck Zone Alarm Spy Blocker. Worked instantly!

PS Message for FF ops: Any chance of keeping this solution accessible for other FF users with similar prob?

I had that for ages too. I fixed it by going into the view drop down menu, going into toolbars and unticking mywebsearch. try unticking those one at a time until one of them disables the black bar.

If you have a problem with a particular toolbar then you should remove (uninstall) that toolbar and not hide it.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Maybe Cor-el could add a short sentence on how to uninstall a toolbar for us non-teccies? It would save time searching the How To's.