Make sure your Flash is up to date not Shockwave. Like MM said they have been quite spotty lately but they have been putting out updates a little more frequently.
If your Flash is up to date and still causing issues go into Tools/Options/Advanced and disable "Use Hardware Acceleration when Available. That should sort out the Firefox issues.
I installed the 12.8's yesterday I believe and made sure my Flash was totally up to date and chanced turning my Hardware Acceleration back on(Had it disabled for the last few drivers because of the issues) So far it's been fine(For now anyway)
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