Unfortunately mozilla doesn't provide a mac version for gecko-sdk, so people have to compile it from mozilla sources. Which is not trivial, as with all big projects.

A good starting point is http://www.iosart.com/firefox/gecko-sdk-macosx/ - the only problem was libIDL. I didn't want to install the entire darwinports just for this package, and compiling from source failed. Eventually I found a binary -

http://www.geeklair.net/downloads/ - and with libIDL and gtk-macos mozilla did compile. Now I'm trying to compile my small jni component against it, I'm curious how hard it is to do jni and play with libraries on mac.