WebKit's native plugin system is fantastic for doing this kind of stuff -- it's expressly for embedding native code as elements that are fully interactive with the rest of the document (though it's awful for implementing the average FF extension). They could use WebKit as the majority of the GUI without having to ship very much inspectable JS.
> though it's awful for implementing the average FF extension
I find that most Firefox extensions are more about modifying the user interface, and less about modifying the content of the page. (i.e. Pearl Crescent Saver, Tab Mix Plus, Download Them All!, Tiny Menu, etc) The big names cover most things you could want to do to the page outside of a Greasemonkey script (Ad Block Plus, NoScript, etc)
* Port iTunes to Webkit
WebKit's native plugin system is fantastic for doing this kind of stuff -- it's expressly for embedding native code as elements that are fully interactive with the rest of the document (though it's awful for implementing the average FF extension). They could use WebKit as the majority of the GUI without having to ship very much inspectable JS.