I have just install mozilla on my box (Gentoo 1.4rc2 (build yesterday from stage1)) Mozilla is working fine but i can't make any applet working ... when i read help->about plugins (in mozilla), it told about flash plugin (i have installed it too and this one works fine) and mozilla null plugin but no informations about javaplugin ... /usr/lib/mozilla/plugins/javaplugin_oji.so exists and is a sym link to /opt/blackdown-jre-1.3.1/plugin/i386/mozilla/javaplugin_oji.so ... so it should work but it doesn't work :)
With a gcc3 compiled mozilla, you need a gcc3 java for this to work. There are numerous bugs about this already. It will be fixed when sun/whoever ships a gcc3 compiled java, else you could compiled it yourself.