Postbox beta 5.52 is using more modern Mozilla code.

The changes necessary are similar to code that already works in Thunderbird as
it has a much more modern code base.

The codebase of Addons needs to be updated to reflect these changes so that
both the old and newer versions of Postbox can be supported.

Created an attachment (id=8384) [details]
1.5 prerelease 148

Last Postbox 5.52 compatible build (works with 5.52 beta6). I am halting
development on this bug, see next comment.

The latest version of Postbox 5.52 beta 7 was  released last Friday, with a big
surprise of removing all Addons support. This means all efforts to be
compatible with the next version of Postbox are pointless as of now. Setting
the bug to WONTFIX until further notice.

I have reached out to Postbox management asking about methods of helping our
users to maintain functionality of our addons. One possible solution (other
than returning to Addons) would be collaboration and including some of the
functionality in the main product. I am currently awaiting their reply.