hi,
i'm assuming that you've installed firefox 1.0 from a firefox installer, ie not YaST, etc..
there are some basics that need to be addressed here, and it applies to any plugin that you wish to use with firefox...
the default install location for firefox 1.0 is, i think, your /home directory and the firefox plugin directory will be in a sub-directory; you therefore need to make sure that you either copy or sym link the plugins that you wish to use into the firefox plugin directory...
the suse mplayer-plugin rpms will (i think) be dependent on a complete suse installation of mozilla. this is ok - you can install mozilla alongside mozilla firefox, then just copy the plugin from the mozilla plugin directory to your firefox plugin directory. not the prettiest way of doing it, but it should work.
the mozilla fire fox website also gives some advice on the installation of common plugins -
http://plugindoc.mozdev.org/faqs/firefox-linux.html
good luck - it's worth it in the end.. and by the way, to make the most of the mplayer plugin, make sure you've got the essential mplayer win32 codecs installed - then you can play back pretty much all streaming media. the plugin works very well.