Checkout
mplayer-plugin , and make sure you have installed
mplayer , the appropriate
codecs, and have read how to put mplayer together before installing the plugin.
If you do not want to compile mplayer, you can download a slackware package of it from
www.linuxpackages.net. You can also find any dependency for it to work if you need one. Once you install mplayer, and it works, you need to create a codes directory (for this particular package) under
/usr/lib as root (i.e. mkdir /usr/lib/codecs), where you will put all the codec files you download from the codecs page.
Once you verify everything works by summoning
gmplayer, then you can compile the plugin, and install the respective plugin files under the mozilla/firefox directories (Read the INSTALL file under the plugin source).
GL
P.S. If you want to make real player work under linux, it would be best to download the real player for linux plugin, instead of using the ones provided by the codecs page.