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.