Debugging Java applets in a Browser on Linux can be tricky because of the way Java caches them. Therefore, if you change the applet, create a jar file, sign and then deploy it, if the jar file has the same name as the old version of your applet, the new version will not get picked up in your browser.
To fix this, you need to remove the old Java applet from your Java deployment cache (browsers do not control where Java applets are stored...).
If you are running Oracle JDK, you first need to open a Java Control Panel window using the jcontrol command:
Start>All Programs>Java>Configure Java
Under Temporary Internet Files, click View...
Linux: From the Java Cache Viewer, right-click the name of the applet to remove, and click delete.
Windows: click Show: Resources; then right-click the name of the applet to remove, and click delete.
Close the window and you are done.
If you want to know where these Temporary Internet Files are stored, click the Settings... tab under Temporary Internet Files. The default location for my version of linux (Fedora) is: