Firefox 3.6 is just around the corner, due to be delivered later this year.
In testing I found out that the old java plugin version (libjavaplugin_oji.so) on opensolaris was no longer recognised and hence java apps didn't work 
So what's the deal?
Since Java 6 update 10, there is a new implementation of the java plugin which means java applets are run in separate Java Virtual Machine instances which are launched by the plug-in's code. Currently they are executed in a JVM instance embedded in the web browser's process.
So what do OpenSolaris/Solaris users need to do?
Install Java 6 update 10 (at least), currently update 17 is available.
Remove the current java plugin from firefox/plugins directory:
rm /export/home/tadpole/firefox/plugins/libjavaplugin_oji.so
Add a symbolic link to the new plugin:
ln -s /usr/java/jre/lib/i386/libnpjp2.so /export/home/tadpole/firefox/plugins
you should also check the system plugin directory: /usr/lib/firefox/plugins/