Past versions of some web browsers have had a serious hole in Java that could allow an applet to execute any machine code on your workstation. This is a huge hole that could allow a malicious applet to do just about anything. For example, your files could be deleted, your mail could be read, mail could be sent as you, or other holes in the system could be exploited to allow someone to gain unauthorized access. Java holes of this type threaten everyone who uses your workstation, not just to yourself.
Enabling and Disabling Java in Firefox
To enable Java in Firefox, click on Tools > Options (on Linux choose Edit > Preferences). Click on the "Content" icon and check "Enable Java".
To disable Java, follow the same procedure, but uncheck the "Enable Java" button.