Whether you're a Mac or a PC user, chances are good that Sun's Java (not to be confused with JavaScript) is installed on your computer, However, chances are also good that you very seldom actually need Java. That's a good thing because Java is fast becoming one of the most targeted third-party applications for malicious exploit via the Web. One easy thing you can do to avoid this method of attack is simply disable Java. When you do need it for that website game or calculator, just re-enable it (but be sure to turn it back off when you're done).
Here's how to disable Java in Firefox.