I don't know who uses those builds, though. We can be sure that the year of Linux on the desktop will never arrive if we expect end-users to unpack tarballs.
Edit: To make that last part a little more constructive, perhaps this will help with distro-agnostic packaging of official Linux builds of Firefox and other applications: https://wiki.gnome.org/Projects/SandboxedApps
Did you try to download the x86 build? If so, it's your fault. If they don't provide an arm build, or if you got the arm build, it's Mozilla's fault, not Debian's.
My contact details are in my profile - I'm happy to help you get this working if you're still interested. It can be hard to find correct instructions for things like manual installs, especially when distros like Raspbian can be slightly different from their upstream sources.