I've recently upgraded Firefox.
This is the Error I receive at YouTube:
Video Player undefined for this type of media (check Tools menu, MediaPlayerConnectivity...)
MediaPlayerConnectivity is a Firefox extension, so you need to update it through Firefox extension manager or get the latest version at https://addons.mozilla.org/en-US/firefox/addon/446/
Then you need to configure it to open flash content with an external player. However, I never had much success playing flash using it.
If you are looking for a solution to play Flash without the flash plugin on YouTube, then there are better alternatives. See the "Flash Replacement" section of http://firefox-tutorials.blogspot.com/2010/05/flash-optimization.html
I would recommend Minitube or my extension FlashVideoReplacer (doesn't play well if you have MediaPlayerConnectivity enabled).
I'll be so dissapointed if I can't keep my install OS Jaunty 9.04 - 9.10 with upgrades, as I would like to do. Seems web is changing so fast, that if I don't upgrade, I'll be stuck with tons of things I don't know how to do ! lol :) I love ubuntu-desktop. I love reading the software in Synaptics that you all do. I know you're all very busy right now, as you like to keep it that way :) I've REALLY enjoyed the amazing experience of being with you all, as a mere user... I cannot thank you enough.