This problem is caused with Firefox 3.6.14. Revert back to 3.6.13 to fix it until mozilla comes out with an update. Link (via mozilla's ftp servers: ftp://ftp.mozilla.org/pub/firefox/releases/3.6.13/win32/en-US/

You might have a problem with your Flash plug-in:

http://support.mozilla.com/en-US/kb/Managing%20the%20Flash%20plugin

If you navigate to http://www.adobe.com/software/flash/about/ it will show whether the plug-in is installed.

just get older version (flash player) than 10.2.152.32 I tried withe 10.1.102.64 and the problem Dissolved to get older version go to (http://kb2.adobe.com/cps/142/tn_14266.html)

I download Fx3.6.15 and the problem happen with the last flash plugin, blocking the cookies to youtube.com and deleting the cookies already stored made posible to play all videos

I had Adobe Flash Player 10.2.152.32 installed and could only hear the sound but no video on YouTube... I resolved the problem by uninstalling version 10.2.152.32 and re-installed version 10.1.102.64... YouTube videos play perfect now Major problem in Adobe Flash Player version 10.2