I was doing some debugging on a Java applet this morning using Firefox 3, and I couldn't figure out how to restart my applet. I was rebuilding my applet and then hitting the refresh button on Firefox, and the old version of my applet was still being executed.
In IE you must press the Ctrl button while pressing refresh (or Ctrl-F5), but this was not working for Firefox.
I finally figured it out: Open the Java Console (available from the Tools menu) and press x which runs the "clear classloader cache" option. Then press refresh, and the newest version of your applet will load.