i have never had luck with xine, i can't recall a time that it has ever worked, and so i use mplayer exclusively. i concede that mplayerplugin crashes mozilla more often than not, and so i removed it in order to keep my blood pressure down.
i get around this by saving media files to a tmp file and then using mplayer to watch them manually.
one thing that takes mplayer from good to tinkle-your-pants awesome is to be sure and install the full codecs package available at the mplayer webpage. sometimes i'm lazy and don't add this in (like on my slackware desktop), and it's still good but not great. iirc, it makes mplayerplugin more stable, but i could be imagining things. heh.
personally, i don't install anything on red hat/fedora unless it's an rpm anymore. so if i can get my grubby fingers on a srpm and tweak the spec file with the configure settings i desire, it is ideal. if this is the case, there's not much to do besides modifying the ./configure line. if you can't get a spec file ... well, there's a 2,000 page manual on rpms out there somewhere you could read.
i think summarizing it is beyond the scope of this post.