What I found at http://forums.mozillazine.org/
You can start Firefox in Safe mode to check if your problem is caused by an extension or theme (switch to the DEFAULT theme: Tools > Add-ons > Appearance/Themes) or if hardware acceleration is causing the problem.
If starting in Safe-mode works then disable your extensions (Tools > Add-ons > Extensions) or use the option in the Safe-mode window) and re-enable them one by one until you find which one is causing it.
Close and restart Firefox after each change.
See http://kb.mozillazine.org/Standard_diag ... ion_issues
Here is the link to the post http://forums.mozillazine.org/viewtopic ... &t=2462477
on the second page they talk about it being caused by adblock filter.
Test it with turning off things one by one and see which is causing it.
If it is your adblock filter then you may need to switch to a different filter or edit the one you have.
After testing come back and let us know what happens. I bet it is one of your add-ons/extensions:ad-block plus
, colorful tabs
, your greasemonkey scripts, real downloader. divx webplayer.