You can no longer package extensions yourself and if you try using "Load add on from file" you get that extension loaded but it's gone after a restart. All extensions have to be signed first to be permanent and Mozilla denied to fix that on their bug tracker.
We're talking about signing by Mozilla to indicate the extension has passed some sort of review process, not signing by the author. It isn't a low bar because it gives Mozilla veto power over what extensions users can install.
