Update: Mozilla now produce intel gecko SDKs so there is no need to use the version I have put here, I’ll leave it for posterity though.
It’s currently a bit of a pain building xpcom components in intel macs. The only officially available sdk is ppc only. Until Mozilla come up with an official version, here is an intel build of it for those that want it: gecko-sdk-mac-intel-188.8.131.52.zip
As the name suggests it’s built against Gecko 184.108.40.206. To the best of my knowledge it’s right but please don’t bug me if you can’t get your component to work with it unless you’re pretty positive that it’s the sdk that’s wrong.
Right now I have no clue how you’d go about making a universal sdk, maybe if you know of a simple way then you could get in touch.