You can very easy enable Java applets in FireFox on Linux. In my case, I use Fedora Core 10 and FireFox version 3. Here are steps how to enable Java applets in FireFox.
# locate IcedTea plugin bash> locate -i icedtea /usr/lib/jvm/java-1.6.0-openjdk-1.6.0.0/jre/lib/i386/IcedTeaPlugin.so # go to the ~/.mozilla/plugins bash> cd ~/.mozilla/plugins # create symbolic link to the IcedTeaPlugin.so in ~/.mozilla/plugins directory bash> ln -s /usr/lib/jvm/java-1.6.0-openjdk-1.6.0.0/jre/lib/i386/IcedTeaPlugin.so
After you restart FireFox, click on "Tools -> Add-ons" and then select "Plugins". You should see IcedTea Web Browser plugin and FireFox should be able to execute Java applets. If you can't locate IcedTea plugin, then you have to install the OpenJDK web browser plugin.
bash> yum install java-1.6.0-openjdk-plugin
If you want to read more or download rpm package, please visit IcedTea main page.