In SM 2.53.17.1, the addons.thunderbird.net version of the extension continues to work.
After updating to 2.53.18, the Mail/news menu entries and Add-ons preferences button do not appear.
The extension compatibility test succeeds.
These source files use nsIPrefBranch2 and nsIPrefBranchInternal but in a way that should be compatible (checking for addObserver beforehand, so would not have run since Gecko 13):
chrome/content/mailredirect.js
chrome/content/mailredirect-debug.js
Something else is amiss. Any suggestions? I'd be happy to fix the problem but the extension has quite a few files.
In 2.53.19, the same applies. The extension can't be reinstalled because of an compatibility check.
After changing the .applications.gecko.strict_min_version to 6 (matching the claimed TB 6/SM 2.3 compatibility in install.rdf), the extension can be installed (!) but it appears as a .xpi archive in the extensions folder while the install.rdf says "unpack: true", matching the previously non-working installation from before SM 2.53.18.
And so, after removing the manifest.json from the .xpi altogether, the extension now installs and runs as before.