It seems that the latest Sun Java release includes a native 64-bit plugin for Firefox. Yay!
I first read about this here: luke.faraone.cc/2009/01/native-64bit-java-plugin-in-ubuntu/
However, the instructions in that article didn't quite work for me on Ubuntu Intrepid 8.10.
Here's what I did:
fakeroot alien jre-6u12-ea-linux-amd64.rpm
sudo dpkg -i jre_1.6.0_12-1_amd64.deb
sudo update-java-alternatives -s java-6-sun
sudo ln -s /usr/java/jre1.6.0_12/lib/amd64/libnpjp2.so /usr/lib64/firefox-3.0.5/plugins/
Then restart Firefox.