Just a follow up before I go to bed, I've taken a look at the READMe for the httpd.js, and a few things pop out...

First, it suggests there's some sort of application wide setup to use the service, and second, it says that httpd.js is not async, only sync. Here's the link: http://mxr.mozilla.org/mozilla-central/source/netwerk/test/httpserver/README?force=1

Does this make anything come to mind that might address what we are seeing?