Addendum for gentoo users
Written by Collins Richey on 19-April-2003.

1. Both mplayer and mplayerplug-in are available as standard ebuilds. I suggest that you get the latest version, which may be masked.

1. become root
2. emerge sync (to refresh the packages list)
3. cd /usr/portage/media-video/mplayer
4. emerge mplayer-....
5. cd /usr/portage/net-www/mplayerplug-in
6. emerge mplayerplug-in-....

... is the full name of the ebuild. (You need the full name to override potential package mask.)

2. Create a link for your mozilla or phoenix plugins directory. For me this is

1. become root
2. cd /usr/local/phoenix/plugins
3. ln -s /usr/lib/nsbrowser/plugins/mplayerplug-in.so mplayerplug-in.so

3. As above, restart your browser and enjoy.