sh /the_path_to_where_you_downloaded_the_script/fix-flash-ubuntu-x86_64.sh
Then the shell-script will run some commands that should fix flash on Firefox on sites as YouTube.
Old script (-> 25th April 2009)
The old script is tested at Ubuntu Feisty (7.04) x86_64.
New script (25th April 2009 ->)
THIS NEW SCRIPT is tested on Ubuntu Jaunty 9.04 x86_64.
Good luck!
If there is any problems, you can contact me here.