Looking a little bit closer, the status on Debian is that they changed what reproducible builds test, and they now test building from different build paths, so that's why the Debian builds are marked as non-reproducible on https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/firefox.html.
Bug 885777 Comment 45 Edit History
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Looking a little bit closer, the status on Debian is that they changed what reproducible builds test, and they now test building from different build paths, so that's why the Debian builds are marked as non-reproducible on https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/firefox.html. They are otherwise reproducible.