sudo apt-get remove totem-mozilla
Now you need to reinstalled mplayer plugin for Mozilla and now your mplayer should work in firefox.
If you don’t want to uninstall totem plugin, you can try this in a terminal
login as root
mv libtotem* oldtotemfiles (move to totem plugins)
Now you need to reinstall mplayer plugin for firefox using the following command
sudo apt-get install --reinstall mozilla-mplayer
This should get mplayer to work.