I’ve found these instructions here:
1. Download the Linux 64 bit rpm from java.com. Ignore the instructions to download the 32-bit version.
2. Follow the instructions on java.com for how to install JRE
3. Exit firefox
4. Make a link to the plugin:
cd /usr/lib64/mozilla/plugins
ln -s /usr/java/jre1.6.0_25/lib/amd64/libnpjp2.so
5. Restart firefox and return to java.com to test