We still want to make the promise to add-ons developers that beta2
will be "stable enough" to start mass converting add-ons, and your
email does mention this.

It follows in my mind that by making this statement we're committing
to keep major functionality after beta2.  For example, it would be bad
if someone wants to take advantage of DOM worker threads in their
add-on, and we pull it before beta3.

I just want to call this out as something that's an implicit promise
we're making.  If we're not ready to make that promise we should make
sure we say so.

--Chris