[WorldMaker]

My outside perspective is that as with any other major project, backwards compatibility and the momentum of old code/bugs.

I don't think they've been "waiting" to solve the UI issues: it's been years of slowly deprecating old systems like XUL, and implementing new systems like e10s. That's been years of anger or at least angst for extension developers, as much backwards compatibility was broken one piece at a time.

The fruit after all that hard work is the new UI.