You're declaring (some of) your event handlers incorrectly:
$('.menuOption').click(function( event ){ // <---- "event" parameter here
event.preventDefault();
var categories = $(this).attr('rel');
$('.pages').hide();
$(categories).fadeIn();
});
You need "event" to be a parameter to the handlers. WebKit follows IE's old behavior of using a global symbol for "event", but Firefox doesn't. When you're using jQuery, that library normalizes the behavior and ensures that your event handlers are passed the event parameter.
edit — to clarify: you have to provide some parameter name; using event makes it clear what you intend, but you can call it e or cupcake or anything else.
Note also that the reason you probably should use the parameter passed in from jQuery instead of the "native" one (in Chrome and IE and Safari) is that that one (the parameter) is a jQuery wrapper around the native event object. The wrapper is what normalizes the event behavior across browsers. If you use the global version, you don't get that.