TIL the reason @Firefox leaks sufficient info (distro, kernel, library versions) to uniquely identify user in its default User-Agent string is that the branding license REQUIRES distros to leak them. UHG, @mozilla, FIX THIS!
To be fair, it looks like Firefox 57+ have reduced the amount of info significantly, but (1) distros should be free to strip anything identifying OS/distro without sacrificing branding, and (2) turning on tracking protection should remove it all.