Hello. I was talking with Firefox devs on this bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1351291 They said, 45 version is old supported release and will end soon. 52 version is stable and will have extended support. If you go to download page of firefox, 52 is downloaded by default.
Firefox-52 is not masked, it is in unstable. We will stabilize it as soon as all build/runtime issues have been sorted out.