hmm, you probably need to install the flash player, did you ever install the restricted extras?
try at a desktop, hit alt-f2 type in 'gnome-terminal', click run
at the prompt type
sudo apt-get install ubuntu-restricted-extras
when prompted enter your password (you will see nothing when you type, this is normal, just type your password and hit enter)
if you havn't installed it will ask you yes/no do you want to install it, type y or yes and hit enter
then when you get back to a prompt (looks like user@user-desktop:~$ ) simply type exit and hit enter, then re-start firefox and see what happens.