I cannot find the firefox.js on Mac OS X, and thus I cannot use Firefox to capture. I'm probably just stupid, but how do I enable ServiceCapture with Firefox/OSX?

OK, I found a way to manually configure Firefox.

1) Find the ServiceCapture proxy address:
System Prefs ? Network ? Ethernet/Airport ? Proxy
Take note of the HTTP proxy address (mine was localhost:8184)

2) Firefox ? Preferences ? General ? Connection settings ? Manual settings
Set HTTP and SSL ports to localhost:8184, set the others blank

But surely there should be a firefox.js for Mac as well?

There is a firefox.js. It is located at:
~/Documents/ServiceCapture/firefox.js

Works perfectly, Thanks! For me this would be file:///Users/myusername/Documents/ServiceCapture/firefox.js

Is this documented somewhere?