(german) ich habe ein macbook pro. wenn ich die grafikkarte umschalte auf intel statt nvidia funktioniert es.

letztendlich habe ich eine lösung gefunden: profil-ordner neu anlegen. das alte profil muss zuerst gesichert werden.

Unter Mac OS X liegt dieses unter "/Users/YourUserName/Library/Application Support/Firefox". Unter Windows im AppData-Ordner. Durch das Löschen des alten Ordners generiert firefox automatisch einen neuen ordner. Die Videos laufen jetzt wieder.

(english) I have a macbook pro. if I switch to INTEL graphics card instead of NVIDIA it works.

I have finally found a solution: Create new profile-folder!

Please backup your old profile-folder first! In Mac OS X this is under "/ Users / your username / Library / Application Support / Firefox". On Windows, the AppData folder. By deleting the old folder firefox automatically generates a new profile-folder and YouTube will play Videos again. ;-)

SOLVED

I went to 3.6.17 and saved my entire profile. That did not fix things. But after I posted here, I got a suggestion from a Mac OSX techie. He suggested something very similar to yours. He said try to use Firefox/Mac OSX as another user. I tried as GUEST. As Guest I was still using the same Firefox install but something was different. Then I deleted the Profile but saved the bookmarks. I then "rebuilt FF" installing the add-ons from before and my saved bookmarks. Curiously, the PLUG-ins from before were not deleted. Everything works OK now.

The key to it all was running FF as GUEST. When that solved things, I knew how to fix it.

The changes made all javascripts work perfectly too. Thanks