There’s a small bug in Ubuntu when you install mplayer-plugin for Mozilla. Two extra links need to be created to get it to recognise DivX files and play them back in a browser. It’s simple to fix. At a terminal type:

cd /usr/lib/firefox/plugins/
sudo ln -s ../../mozilla/plugins/
sudo ln -s ../../mozilla/plugins/mplayerplug-in-dvx.xpt mplayerplug-in-dvx.xpt

Restart Firfox and now you’ll be able to click on a DivX file and it will play properly.

Happy viewing 🙂