I recently installed flashplayer 10. up until then everything was fine. Now you tube, hulu and other video content does not play. I tried removing it and default to flip for mac but the sites continue to ask for the flash upgrade.
A lot of people seem to have this problem, and there isn't any one magic solution, since there are multiple causes for Youtube not working.
I have Real Player installed on my computer, and because of that, there's a extension/plug-in enabled on Firefox. RealPlayer Browser Record Plugin 15.0.2 is the title, and I found it under "Extensions" in the Add Ons Manager. Disabling it allowed me to watch Youtube videos and listen to audio on Tumblr.
If you have RealPlayer, go to the Add Ons Manager and look through "Extensions" and see if you have "RealPlayer Browser Record Plugin 15.0.2" enabled. If so, disable it and see if the problem is fixed.
If not, check out this question right here. There was a lot of useful information.