streaming video
 

A few sites use a script to detect installed media players and well I don't have any of those but I do have Mplayer and Xine, is there some way to tell firefox that I have WM, or QT installed and let Mplayer or Xine take over, and Media Player Connectivity doesn't do it either.

A place that checks like this is Yahoo for movie trailers.

Check out http://mozplugger.mozdev.org/ It lets you use a lot more than just Mplayer.