I think they must be doing something in the beta code. If it was the AddOns themselves I don't see how they could not work for a bunch of daily builds, then all of a sudden they all work again on the same new build. There must be some code they are disabling, then enabling for some reason. I have version checking off in about:config and also use Mr Tech Toolkit.
Doesn't make sense all AddOns would break and fix at the same time. It has to be FF build code. Maybe they use some compile option on milestone builds or something.
Whatever it is, so far other than Windows7 Jump List, I haven't seen a single new feature in 3.7pre anyway. So there's no impetus to use it.