We might be resolving symlinks to their physical paths, making each download look like a new install. If so, the nixpkgs issue I mentioned above is probably more relevant than expected.

I'm not entirely certain, but if you want to source dive, nsXREDirProvider::GetInstallHash is a good place to start: https://searchfox.org/mozilla-central/rev/6c9f60f8cc064a1005...