Does that happen in safe mode? Maybe it is hardware acceleration causing that problem? It might even be an extension causing it. I am using the same version of Mac OS, the same version of Firefox, and the same version of Flash, and I am not seeing that on my computer.

Try to delete the pluginreg.dat file in the Firefox Profile Folder and maybe addons.sqlite as well to reset the plugin registry databases.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder

• http://kb.mozillazine.org/Profile_folder_-_Firefox