It's possible that the httpd.js we're using needs to come from Mozmill, and might need to be slightly different than from the documentation I originally sent you. :/  Sorry about that.

Here is an example of httpd.js being used successfully in a Mozmill test: http://mxr.mozilla.org/comm-central/source/mail/test/mozmill/shared-modules/test-cloudfile-yousendit-helpers.js

Note where we're importing from ('resource://mozmill/stdlib/httpd.js').

Instantiation: http://mxr.mozilla.org/comm-central/source/mail/test/mozmill/shared-modules/test-cloudfile-yousendit-helpers.js#134
Initialization: http://mxr.mozilla.org/comm-central/source/mail/test/mozmill/shared-modules/test-cloudfile-yousendit-helpers.js#147

httpd.js should be asynchronous - the bug that the readme refers to has been marked RESOLVED FIXED. Outdated documentation is the worst. :/

So try importing from that new location, and see if that makes a difference?