Can we turn that around?  Would it be possible, given an interface, to scan AMO
for binary extensions that use that interface in their binary blobs?

The problem is that API changes are not the end in and of themselves, usually.
They are often needed, for our core "internal" APIs to address other issues that
come up.  No one is suggesting changing APIs for the fun of it, I don't think.

For example, note that if a blocker issue comes up in RC1 right now that can be
most easily fixed by changing nsIContent we would change nsIContent before we
ship final.  The question is whether 1.9.1 should be held to a higher bar than
this (as 1.8.1 was).

-Boris