hmm might it be that Mozilla did install the java (into your system) but didn't copy over the needed plugin file..? (if Plugins in your Mozilla's Help-menu doesn't show up any javasupport, then at least the file is missing).
and here's the guide on what to do, when you have installed java into your system, and you need to get your browser to "see" and use it:
1) if needed, update database and then locate the file we need ( libjavaplugin_oji.so) and browser's (mozilla here) plugin directory; in terminal, type
locate mozilla | grep plugin
first command updates database so we can find the files. the second one finds the java file we need (it can be, for example, in /usr/java/j2re1.4.2_03/plugin/i386/ns610-gcc32/ but this may vary) and the third one tells where the plugin dir of your browser is.
2) next (still in terminal) cd into your browser's plugin directory (path was located above):
where you change the path to the one you found with locate.
3) when you're in it, symlink the .so file there as follows:
ln -s /the/path/to/the/javafile/we/found/libjavaplugin_oji.so
where the path is the one you found with locate above (the whole command could be like ln -s /usr/java/j2re1.4.2_03/plugin/i386/ns610-gcc32/libjavaplugin_oji.so if the file was in the given directory - it may vary depending on your settings so use the one you got with locate).
now after this we should have symlinked the libjavaplugin_oji.so to the plugin directory of the browser. next step is to close your browser and re-launch it - now open Mozilla's Help menu and see Plugins (this works for mozilla, but with other browsers you just have to see if all is right) and you should see line(s) telling you have java.
4) surf to a page which has java inside, and see if it works
it really should.
EDIT: this is what the mozilla-dudes tell in their webpage: "Mozilla 1.4 and later, and Mozilla Firefox, are compiled with gcc 3.x. A gcc 3.x compatible version of the Java plugin must be used. JRE 1.4.2 contains a compatible plugin." it means that if you use M1.4 or Firefox, you need JRE1.4.2 or later or it won't work
hopefully you understood what I wrote...it got a bit long. but basic steps are just to find the file and symlink it to browser's plugin dir - easy, huh?