Since a short time flash content does not work in my Firefox browser any more. Installing the actual flash plugin from here, doesn't handle the problem either.
Finally the Firefox FAQ helped - you have to disable OBJ-TABS in the Adblock plugin:
1. Open the Adblock preferences (Tools -> Adblock -> Preferences in Firefox)
2. Click on "Adblock Options", and untick "Obj-Tabs"
Here is a useful link to test you flash-player plugin!
