"In theory, you could have QueryInterface call getInterfaces() and do its lookups based on that, but it probably isn't necessary."

This is how we implement QueryInterface on all our components in the Forecastfox extension. We also have a component exposed to content so our icon packs can be installed from the web. See: http://www.ensolis.com/cvs/forecastfox/src/chrome/content/forecastfox/icons/web-service.js

Great series so far. Suits my needs perfectly. Although currently non-chrome scripts need to request privileges to access the exposed object, I assume that is what your next post will resolve. Am eagerly awaiting your next entry to this series.
Thanks!
James