Comment 58 for bug 173966

go to /usr/lib/firefox-addons/plugins$

and do:

sudo ln -s /etc/alternatives/firefox-javaplugin.so

I am using firefox beta 4 and ubuntu gutsy 7.1 my applet works already. If you are seeking to get your java up on firefox

if you cannot find firefox-javaplugin.so maybe you can try
ls -l | grep firefox
to get all the files that are related to firefox
probably one of them is the firefox javaplugin link

just swap my given firefox-javaplugin.so for that file that you found and try if it works.
hopefully this helped
Thanks to those who posted above that leaded me to /etc/alternatives and /usr/lib/firefox-addons/plugins I would never have figured out where the plugins are myself