IIRC, this is happening because of the complete internal rewrite - the relevant APIs to expose to extensions simply don't exist yet, and they're implementing them to get the most-wanted extensions available first.
They could use better messaging on this so people who are on firefox precisely for this reason don't think that something has fundamentally changed.
A statement along the lines of "We're working with the most popular Extension creators to get up to speed on the mobile extension API" (which could mean anything) would have been better.
