First, see Flash Plugin - Keep it up to date and troubleshoot problems and test your Flash plugin by going to http://www.adobe.com/software/flash/about/

For other ideas, see

• Fix common audio and video issues
• http://kb.mozillazine.org/Flash#Troubleshooting

I also have this problem since updating to 6.01, have tested flash player on the Adobe site, and have also updated flash player, but still have this problem. Works fine in IE, and worked fine before updating to 6.01

I have found a solution that worked for me.If you clear your recent history, Go to Tools-Clear Recent History make sure the first 4 are checked, uncheck the others. This worked for me, and I can now watch youtube videos again.

It didn't work for me.

Did you clear your cookies and cache as explained here? Basic Troubleshooting - Clear your cookies and cache

If that didn't help then you can try these other suggestions, from the Fix common audio and video issues article:

Disable extensions
Sometimes a Firefox extension can block plugins or otherwise prevent audio or video content from playing. If the video or audio plays when you disable all extensions, then one of your extensions was causing the problem. For more information, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Other solutions:

• Verify that cookies are enabled in the Settings for privacy, browsing history and do-not-track window. Verify that the site is not blocked in the Cookie Exceptions list.
• Allow the website as an exception in the Firefox pop-up blocker. The "Block pop-up windows" option is found in the Content panel of the Options window.

If you still have problems playing videos and it's only happening on YouTube's site or with videos that link there, go to the YouTube Help Center at http://www.google.com/support/youtube/ and do a search for your problem (e.g., "YouTube videos won't play"). Also check the "Help resources" section on the bottom of that page.

Disabling the Ad Block Plus plug in worked. I can now watch YouTube videos. Hmm...I wonder why that plug in was blocking YouTube videos anyway. Thanks for the help

Glad to hear you got YouTube videos working. I'll mark my post that mentioned disabling extensions as the solution.

On why AdBlock Plus is blocking YouTube videos, you can ask on the AdBlock Plus forums, https://adblockplus.org/forum/index.php ...or search the forums for YouTube.

I had to disable Tax Mix Plus after upgrading to Firefox 6.01. (was on FF5 before). The Adobe Flash videos in YouTube just flash in and out. Sometimes you see the video, other times it goes black. If I shake the mouse I can sometimes get it to come back from being black, but only for a 1 second. Hope this helps!

Actually I discovered I could use Tab Mix Plus, but once I went to 3 rows of tabs is when the YouTube videos would flash in and out. Once I closed some tabs the problem went away.