I just discovered this problem with both IE8 and Firefox. Adobe is aware of the issue: http://forums.adobe.com/thread/872999?tstart=0 I followed the instructions in the above link and at least got videos working again in IE8, it says this also works for IE9 but I didn't try that out. To make youtube videos work in firefox I put an s in http: to make the URL read https: and that allows the videos to play. The other working fix I found is to clear the offline storage cache by going to Tools > Options > Advanced >Network > Clear Now button. After clearing the cache and restarting firefox youtube now works.