I had the same issue and it seems Adobe Flash Player 10.2 is the Culprit. Revert back this version and it all seems to work properly again.

Thank you! I have been trying to fix this problem all day. Installing the previous version of Adobe Flash Player fixed it. You Tube works fine again.

For me it was not necessary to make any changes in the plug-in-list. I just deleted all cookies from youtube.com and started again. And then I could watch the videos on Youtube again.

Update: Sorry! Sorry! It only worked once!

I had the sam problem yesterday. What i've done is, that i downloaded "Uninstall Flash Player tool" (http://download.macromedia.com/pub/flashplayer/current/uninstall_flash_player.exe). Than i ran it, and follow the instructions. After that, make sure that Flash player is removed from your computer (open Youtube - it should not work).

After that i downloaded older version of Flash and installed it (**notification** - all old versions do not resolve the problem).

I installed this version of Flash [10.1.53.64] http://www.oldapps.com/flash_player.php?old_flash_player=19?download

Mozilla version: 3.6.15

Problem solved for me! I hope this would be useful for you. Good luck!

I tried installing an old version of Flash, but the problem persists. Everything worked fine up until 3.6.15.

Update: Disabling Adblock Plus on YouTube restored the video. (It also enabled ads on YouTube, which is supremely frustrating.)

try clearing all cookies from your browser that should take care of it

Clearing all cookies works *ONCE*. Yes, ONCE. Unless you block cookies, in which case, you can't log in.

Why is mozilla not fixing this?

im almost positive that it's the flash plugin because this issue occurs on both firefox and google chrome. (both use flash version 10.2.152.32)