We have the same version of Java and Mozilla there sasquash...
Maybe you made the link into the wrong directory, because I noticed there are 2 directories named mozilla...
One is mozilla and the other one mozilla-1.2.1
For mozilla 1.2.1, the directory for plugins is /usr/lib/mozilla-1.2.1/plugins
Well I don't see any reason to upgrade to Firebird just yet. Any suggestion why I should? Mozilla 1.2.1 is good enough I think. They also have a pop up stopper extension for it.