One reason for add-on signing is that there's a major attack on Firefox using stolen add-on IDs to insert malware. This started in early 2015, and one of my add-ons has about one-third bogus versions out there. They all have different random version numbers, typically above 1000. Those are about to get flushed out of the ecosystem.

Whether add-on signing is part of a power trip at Mozilla to enforce a walled garden with Mozilla-favorable policies remains to be seen. When Mozilla blocks an add-on which disables Pocket or Sync or Hello, we'll know that AMO has turned to the dark side.