I've had that a few times. Did you mean you updated Firefox to 5.0?
Try Firefox in Safe Mode. If you are in Firefox 5.0 starting safemode is done by going to HELP then Restart with Addons Disabled, then ok and it will restart. It will ask what you want to make permanent, dont check any of the boxes just continue with safe mode. Try Youtube and see if it works now. If Youtube works now then its an issue with one of your addons. Restart firefox and disable one of the addons and restart the browser. If the problem goes away the problem was due to that addon. If disabling that particular addon didnt cause youtube to work, then disable the next addon and try again, continue until you get youtube to work properly.
If that didnt do it then I would run CCleaner to clear all temps. After that, then open Firefox, Tools, Clear Recent History, "Time Range to Clear" EVERYTHING and under DETAILS select all boxes so everything is cleared.
Then uninstall Java and Flash (check installed programs list to be sure all old versions of both are removed) and reinstall the most recent version of both.