did you notice a xine plugin too? if so, take it out.
and/or take xine out all together, reboot and try to see if firefox pulls up mplayer then.. maybe this will 'reset' things.
this is why i use the ol grandaddy, firefox
it has seperate settings for everything, like what programs to play when different extentions are encountered.
i'm sure firefox has this configuration 'somewhere' but i'll be darned if i know, because the last time i messed with:
:config (which you can type in the url box and pull up) it would not for the LIFE of me save the settings between sessions.
anywayz.. keep us informed.