This actually doesn't sound that hard to do, because it looks like others are doing it (apparently "Second Life" does) but I have yet to look at the Firefox code.
I think there is two ways to approach this:
(1) Try to use the Firefox source code as a framework that you link to and either access or create your own NSFireFoxWebView class, which of course would use the Gecko engine instead of WebKit.
or
(2) Launch Firefox invisibly in the background and Distributed Objects to take the Firefox NSView, put it into your application and sync it with the Firefox app running in the background.
I would post this question on Apple's "Cocoa-Dev", "Webkitsdk-dev" & "Web-dev" lists or better yet just ask the Cocoa Firefox developers what they recommend:
http://lists.apple.com/mailman/listinfo