Unsigned addons are rejected, xpinstall.signatures.required is ignored

Hello,

to reproduce, I downloaded some official xpi file from the AMO. Using it for "Install Add-on From File..." at about:addons works great. Then I changed some contents inside the zip and tried again. Now it fails with this add-on could not be installed because it appears to be corrupt. Toggling xpinstall.signatures.required to false does not help! When I change the content back to what it was before (also by editing the archive), it works again, so it can't be a bundling error, but a checksum/signature issue. Sideloading via about:debugging#/runtime/this-firefox's Load Temporary Add-on... works in either case.

This entire behavior is 1:1 how Firefox itself works since many years apparently: https://bugzilla.mozilla.org/show_bug.cgi?id=1298806 xpinstall.signatures.required is only respected in nightly builds for some nonsensical reason (Screw you Mozilla!!!)

Motivation: Developing and installing add-ons locally without being forced to upload them to AMO.