Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
To Enable SafeMode
You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
Once you get the pop-up, just select "'Start in Safe Mode"
If it works in Safe Mode and in normal mode with all extensions (Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears.
Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: "Firefox > Quit Firefox"; Linux: "Firefox/File > Quit")
Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache.
Note: This will temporarily log you out of all sites you're logged in to.
To clear cache and cookies do the following:
Go to Firefox > History > Clear recent history or (if no Firefox button is shown) go to Tools > Clear recent history.
Under "Time range to clear", select "Everything".
Now, click the arrow next to Details to toggle the Details list active.
From the details list, check Cache and Cookies and uncheck everything else.