Hi! I had this problem too. When I started Firefox from the terminal, I could see there was a problem with the gecko-mediaplayer. So, here's what solved the problem for me:
After that, I rebooted (just to be sure) and reinstalled both:
sudo apt-get remove --purge gecko-mediaplayer gnome-mplayer
The bug is documented here: https://bugs.launchpad.net/ubuntu/+s...er/+bug/812053
sudo apt-get install gecko-mediaplayer gnome-mplayer
For some reason, upgrading from natty to oneiric failed to make a "clean" installation of the gecko-mediaplayer.