The code changes in Postbox latest betas 4.52 make some code changes necessary
that 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=8370) [details]
3.5 prerelease 51

First test version that switches to the modern code in modern Postbox builds;
this effectively fixes filter creation and merge process for Postbox betas
5.52.0. I am trying to maintain backwards compatibility, so one of the switches
used in chrome.manifest is:

content shimQuickFilters chrome://quickfilters/content/shimEcmaOld/
application=postbox@postbox-inc.com 
content shimQuickFilters chrome://quickfilters/content/shimEcma/
application=postbox@postbox-inc.com platformversion>=26

First line uses a path with js module containing the specific alternative code
that was used for the (outdated) JS engine of "legacy" Postbox. It is
overwritten by the second line in modern builds, so that it also uses
constructs like for..of which caused old Postbox to throw an exception even
when the code was not executed. It would basically make the complete JavaScript
module containing this code invalid / unexecutable.

Created an attachment (id=8383) [details]
3.5 prerelease 70

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. One possible solution (other than returning to
Addons) would be collaboration and including some of the functionality in the
main product. I am awaiting their reply.