Sometimes, usualy when i'm in another tab in firefox and i return back to the tab with the video, i only see a gray area instead of the video. This also happens if i switch to full-screen and back.
The problem can be corrected by either switching tabs again or switching full-screen mode.
What version of mplayerplug-in are you using? Also you might try setting vo=x11 in the mplayeplug-in.conf file.