Dude, just install JRE (you can download this for free from Java website - just google). Read the instructions on how to install that; the instructions are given on the same website. Now, you just have to find the Firefox plugins directory and give the command:
ln -s /path-to-java-plugin-folder/lib-java-plugin.so (check the filename)
Of course, you need to be in the firefox plugins folder. Or if you know how to use ln -s, problem solved. Restart the browser now.