I was using the Firefox 4 beta for a while, and Youtube had always worked. Then when the final RC came out (or one of the last betas, don't fully remember), I immediately installed it, yet Youtube gave me the message stated above when trying to play any video. I have even tried switching to the HTML5 trial to no avail. I have been having to downgrade to the latest 3.6 version to be able to view Youtube videos.
Clear the cache and the cookies from sites that cause problems.
"Clear the Cache":
"Remove the Cookies" from sites causing problems:
Forgot to mention that I have tried that a few different times and did not solve the problem.
I've also tried uninstalling then reinstalling the flash plugin like some have suggested when google searching, but that also did nothing to solve it.
Thanks for the reply however!
Start Firefox in Troubleshoot Firefox issues using Safe Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).
If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
In Firefox 4 you can use one of these to start in Troubleshoot Firefox issues using Safe Mode:
You can also try "Reset all user preferences to Firefox defaults" on the Troubleshoot Firefox issues using Safe Mode start window.
I tried all the suggested items listed and none worked. Even in safe mode youtube videos still gave the same error message in Firefox 4.0.1.
I have yet been able to compare with another Mac with Firefox 4 installed, hopefully I will be able to do that soon to see if I have anything different in the settings and such. If not, it may for some reason be a hardware issue (?) or possibly some settings in Snow Leopard that are messing it up versus it being a problem with Firefox.
Removed and reinstalled Firefox... problem solved.... Also got rid of the "another version running" flash screen on startup and updated to v13...