ESR52 codebase is incompatible with anything below Universal C Runtime (CRT) in Windows
First, Mozilla fixed VS2015 compilation https://hg.mozilla.org/mozilla-central/rev/e597b8795fbe Second, Jacek "fixed" ffvpx compilation on mingw. And it became incompatible with UCRT on mingw (Jacek, don't do that again!) https://hg.mozilla.org/mozilla-central/rev/5680a55b2ec1 Third, Mozilla dropped support for building with Visual Studio 2013 https://bugzilla.mozilla.org/show_bug.cgi?id=1186064 in Firefox 50. But not only that! They started vigorously removing all their patches for it! https://bugzilla.mozilla.org/show_bug.cgi?id=1277155 and almost all other compatibility shims/workarounds were removed before ESR52.