Since FireFox 3.x, JSAppletInitialized() callback seems broken under MacOSX only (no problem under Firefox/Windows or others browsers). If you need this callback then here is a workaround:
Add the following parameter to JavaScript: <PARAM NAME="forcejs" VALUE="false">
It forces the applet to use another API to Java<->JavaScript communication. However, it is not recommended for other browsers and operating systems. So, could you try to update your JavaScript to have something like:
....
if (browser == firefox3) && (os == macosx))
{
document.writeln('<PARAM NAME="forcejs" VALUE="false">');
}
...