Extensions have some sort of version tag in them that indicates which version of the browser they're intended for. This, in theory, prevents incompatibility problems.
In practice, it means that every extension author has to scramble to test and 'recertify'/modify their extensions for new builds, no matter what. (Beats having chunks of old extensions active or breaking things 'invisibly,' but then... as of 0.9, that could still happen.)
I expect there's fair room for improvement, but they seem to be taking the approach that, *in theory,* the bloat of backwards-compatibility itself can be pushed out to an extension... which could even be a valid philosophy, if someone were maintaining one. (More likely they'll try to stabilize the API across the 1.0 series, and hope someone gets around to further overhaul of the extensions system before whatever the next major will be - 1.1 or 2.0?)
