[cyphar]

In openSUSE our build system can be configured to auto-check the signatures of the source archives used for building, so you can check the builds to make sure that we are building from an official source releases (assuming the GPG detach-sign their source tarballs -- something I recommend any software release manager do).

But most distributions do their own builds, and without reproducible builds being universally available -- not to mention that distributions usually have minimal compiler hardening flag requirements as well as patches in some cases -- you couldn't reuse signatures for the final binary. Also the entire package is getting signed, so the binary's signature wouldn't be sufficient (and checking it on-install would be quite complicated as well).

> Still, it would be weird that, as far as Mozilla trust goes, a signed exe from internet is better than a signed package from your preferred package manager.

I think that has always been the general case, since distributions are an additional layer of people maintaining a downstream copy of a project. But don't get me wrong, most distributions have processes that allow you to verify that the source used for builds of large projects like Firefox are built using the real sources.

There's also usually several layers of human and automated review before a package upgrade can actually land in a distribution.