+++ This bug was initially created as a clone of Bug #1520177 +++

Although Bug 1520177 committed several necessary fixes, and seems to have fixed the issue (mostly) for the esr60 mingw-gcc build; we still need to investigate the mingw-clang build on esr60 and -central.

The issue is not gcc vs. clang but it seems the problem is that a) debug builds hit assertions (regardless of being compiled with gcc or clang) and b) the support on non-debug builds is not on par with that provided by vanilla Firefox (again regardless of compiler being used). Let's keep bug 1520177 for the remaining work/investigation.