I think this error happens when you use Firefox + you have opted-in to the YouTube HTML5 Trial + the video hasn't been converted to WebM. To fix this:

1. Go to http://www.youtube.com/html5
2. Click the Leave the HTML5 Trial link at the bottom of the page.
3. Reload the video. It should now play using Flash.

It didn't work for me. :-(

The same happened with me!! I just downloaded Chrome. I prefer FireFox, but I don't have the patience to wait for Mozilla to fix their obvious problem. I'll check back later.

the problem caused not by your browers or youtube infact,I fixed the problem with some guys method and work very well,I think fixed it from the root,now whenever and whatever video on youtube I can watch it no problem, thx the guys share,you can check the method in his video description it will help you.

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This is how I solved this problem: open firefox, then go to Tools then add-ons. there you choose plug-ins then you find shockwave flash add-on then disable the plug-in. then you go on to download adobe shockwave for director plug-in then you install it and make sure you disable the basic shockwave flash add-in. You restart firefox and enjoy the videos. To download the shockwave for director plug-in got to this page and read carefully how to download the right plug-in. If you respect all the step you should be able to watch video and others on your firefox browser.

Well, this might be a funny situation, and I thank all of you who have responded to this, but since I upgraded to Firefox 14.0.1 the problem with YOUTUBE has stopped. That is not to say that others might or might not find the same outcome, just that I don't have the problem anymore.

I have had that problem also. I removed most recent version of Adobe and then downloaded one from the archived versions that was not so up to date. All video now working fine. One concern, however, is that Adobe will want to download most recent version of Flash and I hope I can override it.

However it happened, the issue resolved itself. I now can view You Tube videos in Firefox without problem.

WTH:

I finally figured out the problem after puttering around a bit and doing different add-ons and downloads. What is this? a 9 to 5 or the internet? Why should this have to be work for me? I am not a computer technician. I found out it is all Firefox's fault because they chose to disable a Realplayer something or other. And without even notifying anyone, making us think it is our fault; doing all this downloading and other crap. You, know what, I am going back to good ole reliable IE, that's right! INTERNET EXPLORER! I never had these problems with them.

Thanks a lot freaked-up fox. you have a happy and a healthy : (

Yes I figured this out too. I am a diehard Realplayer Fan. So if something has to go it's going to be Firefox. I originally thought it was a an ADOBE issue. There is very little I can download on realplayer now. But the very little is possible if you downgrade to Adobe Flashplayer 10.

And I can view the You Tube Videos

I still can't watch all videos with firefox (almost all Youtube videos but only almost, not all non Youtube videos work either...). I prefer Firefox to IE (noticeably for color management) but I switch back to IE which works fine.

Do you know how to be informed when Mozilla will have completely fixed the problem?

NOTE

1. some problems may be due to using the YouTube experimental feature to utilise HTML5, if so turn that off as explained in a post above
   • some users may be using HTML5 as a workaround due to Flash Player 11.3 problems.
2. FlashPlayer 11.4 has recently been released and is expected to solve some problems, go to /plugincheck and upgrade FlashPlayer to 11.4 that may resolve the issue.

If you have continuing problems please start a new thread by asking a new question using /questions/new.

@zukadizajn .Excellent ! this was totally helpful. and short immediately solution ! MANY TANKS !

Hello everybody, I am a web-designer and when I embed videos on some of my sites I keep getting this error, I don't know what to do, you can check live stoixima or Betting sites but i f I turn off HTML5 everuthing seem to work ok !

Hello friend, me too experiencing the same problem while playing you tube videos. I'm unable to play videos at all. I'm facing this problem for the past 4 months and unable to solve it. Can you help me out in this one.

Thanks

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