First of, huge thanks to Kilz for the amount of follow up. I've seen quite a number of communities before but I can't say I've seen any as supportive as this.
Next, I used to have a problem with mplayer buffering up to 99% and then just sitting there, whenever I'd hit rightclick -> play it would stop. So I tested a few things and ended up getting it to work as follows.
You should first check if you have both the 64 and 32 mplayer versions, or this won't work, so check by doing:
Then, if your result looks like this:
ls -al /usr/bin/mplayer*
Do the following:
-rwxr-xr-x 1 root root 8212848 2006-10-13 03:25 /usr/bin/mplayer
-rwxr-xr-x 1 root root 7251160 2006-06-04 22:31 /usr/bin/mplayer64
Restart firefox and test. This did the trick for me, hope it helps anyone else out there.
sudo mv /usr/bin/mplayer mplayer32
sudo mv /usr/bin/mplayer64 mplayer