Any idea why many or most Youtube videos do not play for me in Ubuntu?

I would have thought I would be asked to download a plug in or something but I am not. I am using Firefox.

You need the Adobe Flash Player plug-in to watch YouTube videos. [ Download | Instructions ]

Other resources:

Installing Flash on Ubuntu

sudo apt-get install ubuntu-restricted-extras

What version of Ub are u running ? Go to System > Adm > Synaptic Manager type n Adobe in Search Box

Firefox check plugins Tools > Addons >Plugins

If you have the 64bit version on ubuntu then read and then run the script at the end of the 1st post and it will work perfect http://ubuntuforums.org/showthread.php?t=1358591 (well it does for me)