Some Ubuntu (hardy heron) users have had problems getting Java applets to run correctly once they upgrade to Firefox 3. If this happens to you, the fix is to to remove the icedtea-gcjwebplugin package from your system (this is a browser plug-in to execute applets, and it seems to have compatibility issues with Firefox 3). To remove it, open a terminal window and enter the following command:

sudo apt-get autoremove icedtea-gcjwebplugin

You'll may be asked for a passowrd. Once this completes, close Firefox and restart it. You should find that Java applets are now working.