Crazy idea that doesn't entirely fit in with this spec: We could provide an API for external editors/IDEs to use to load the add-on source files as a project (plus things like requesting the add-on be reloaded), and develop plugins for popular editors/IDEs to support this feature.

Even when we eventually get a full IDE in Firefox's devtools, this would be nice to have - both for add-ons and for web development. We should talk to the devtools team about this - it could be a nice easy way to bootstrap a more integrated developer workflow, without actually needing to develop a full IDE ourselves right away.