Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

• You can open the Firefox 4.0+ SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
  

Don't select anything right now, just use "Continue in SafeMode."

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

When you figure out what is causing that, please let us know. It might help other user's who have that problem.

In Firefox 4 and later Safe mode disables extensions and disables hardware acceleration.

• http://kb.mozillazine.org/Safe_mode

Try to disable hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

If disabling hardware acceleration works then check if there is an update available for your graphics display driver.

@dbonneville: try disabling Read It Later Addon, this was causing the problem in my case

I, too, had this problem and it was indeed caused by the "Read It Later" add-on, however, the folks at "Read It Later" seem to have already updated their add-on to fix this issue. Ensure you're running the latest version of the "Read It Later" add-on (you can manually update by going to FireFox > Add-Ons > Extensions tab then click the cog in the top right corner and select Check For Updates) - My add-on shows as version 2.1.4 last updated 3rd January 2012).

This should fix the problem.