They do have a stable SDK with forward compatibility now, just most extensions were not written to use it as they were written before it was released. Like Chrome's API, it's less powerful than the XUL addons, but it covers what 95+% of addons need.
IMHO they should have released the SDK first and pushed for developers to use it, and then started rapid release, but that's not how they did it. ;-)
It's a legacy problem like that many other systems have. Getting big enough that people bitch about some of the early imperfect decisions you made is a problem many devs would like to have. ;-)