Videos not playing with mplayer & player-plugin

[R L]

I have MPlayer 1.0pre6-3.4.2 and mplayerplug-in-2.80-13.1.  When I
play videos on Firefox, I get no video and when I play it manually
through mplayer command line, I get:

The selected video_out device is incompatible with this codec.

FATAL: Could not initialize video filters (-vf) or video output (-vo).

I mainly want to play videos in firefox. What's up?