Well, today I finally found a resolution for Flash 8 and Firefox not working correctly. The symptom is that, with Flash 8 installed with Firefox, you are unable to display any Flash enabled content. You get no placeholder, just blanks where the content should be. The resolution? Turn off the Adblock extensions’ Obj-Tabs feature (it’s what puts the translucent “Adblock” tab off each Flash animation). As soon as you disable that feature, Flash works perfectly.
To disable this part, click Tools -> Extensions. Highlight Adblock, click “Options.” In this new window, click “Adblock Options.” Find “Obj-Tabs” and click it to uncheck it. You’re fixed!