I'm sorry to have posted my earlier comment without enough investigation on my part. In fact the Flash Player 8 issue was due to a conflict with Adblock extension. I just found this on Firefox site and it works:

Flash Player 8.0 does not work if you use Adblock with OBJ-TABS enabled

Due to increased security in Flash Player 8.0, enabling OBJ-TABS in Adblock will result in no Flash content being displayed. The only workaround at this time is to disable OBJ-TABS.