I know this thread is really old, but I have finally got a solution, so I thought I would post it to share my excitement and for anyone else who needs a solution.

I could not get movie clips and such to play for the life of me in mozilla. I tried virtually everything.
It was only recently that I was able to get them working.

The answer: I used apt-get to install the mplayer plugins for mozilla:

apt-get install mozilla-mplayer