Finally, finally, FINALLY! The Sun Java plugin now works on Firefox amd64 in native 64-bit. It has already been included in Ubuntu 9.04 Jaunty, but the packages work just fine on Hardy as well, and probably on Intrepid.
sudo dpkg -i sun-java6-bin_6-13-1_amd64.deb sun-java6-fonts_6-13-1_all.deb sun-java6-jre_6-13-1_all.deb sun-java6-plugin_6-13-1_amd64.deb
sudo apt-get remove --purge icedtea-gcjwebplugin
Then restart Firefox and Sun java will load natively 64-bit. Check it:
tyler@baal:~$ java -version
java version "1.6.0_13"
Java(TM) SE Runtime Environment (build 1.6.0_13-b03)
Java HotSpot(TM) 64-Bit Server VM (build 11.3-b02, mixed mode)
I’ve included them in the repository.
Update 2009-07-16: A more recent version is now available in the hardy-updates repository. I have removed the above copy from my repo. Intrepid users should upgrade to jaunty anyway, but can still download packages directly.