The very best way should be installing both JRE and Mozilla/Firefox from YaST. SuSE includes both packages and if both are installed, java should work.
However, if you want to install the plugin manually(with JRE already installed through YaST), you need to find where the package is installed and then create a symlink to the Firefox plugins folder. e.g:
ln -s /usr/local/<your JRE path>/plugin/i386/ns610-gcc32/libjavaplugin_oji.so /usr/local/firefox/plugins
If you want to do everything manually (installing both JRE and Firefox plugin), take a look into this thread:
it's meant to help installing Limewire, but should help installing java plugin as well (step #4).
Well, unless you _really_ need the latest Mozilla or Firefox, I'd stick with the packages shipped within SuSE's CD's, for both JRE and Firefox, but that's your call