This may be caused by a problem with a theme or an extension. See if the procedures in the following link helps - https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Issues with a 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.

You can see the toolbars in View > Toolbars.

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

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

This problem started when the browser was upgraded to beta 11. It was not present in beta 10. The problem occurs when the bookmark toolbar is displayed, and tabs are on the bottom. A number of other people are having this problem as well. I know it occurs in Vista and Windows 7.