Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Where is the black area top or bottom, is it is at the top in the toolbar area, try

1. "Alt+V" > toolbars > ...
2. un-check all toolbars
3. if the problem goes away, re-enable toolbars one at a time to find which one
4. "Alt+V" > toolbars (or "T") > ...

You say Firefox page, is that a specific page (url please), all web pages, or were you referring to toolbars.

The add-ons bar is at the very bottom of the page and is a toolbar.

Can you attach a screenshot if you still have the problem?

• http://en.wikipedia.org/wiki/Screenshot

Use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed the maximum file size (1 MB).