Firefox core, extensions, and so on
It ought to be possible to implement the needed callbacks in and out of Postbox using the modern Browser Extensions API. Of course, the details of what would need to be supported and so on would have to be figured out, but form a strict technological perspective, I don’t see any reason it wouldn’t be feasible…
/me wanders to the Postbox site to drop a note…
Eric Shepherd
Senior Technical Writer
MDN Web Docs
https://developer.mozilla.org/