First let's test if Java works. Goto http://java.com and click on this link "Do I have Java?" and Verify Java version. If you see something like this, then you have Java plugin enabled.
How to disable it?
Goto Firefox -> Add-ons -> Plugins -> Java(TM) Platform plugin and select to disable it.
Now go back into java.com and see if Java works and now you should see that Java is not working, so you successfully disabled it.