By default, when running firefox via firefox.exe /console or firefox.exe /attach-console,
traces logged to stdout wouldn't work unless they are from the parent process.

This relates to bug 1189223.
A workaround exists, it is about running firefox from PowerShell, like this:
.\firefox.exe | Tee-Object CON

But it would be nice to have this working without any workaround.

Note that the JS Tracer relies on dump which logs to stdout like this:
https://searchfox.org/mozilla-central/rev/31f5847a4494b3646edabbdd7ea39cb88509afe2/js/xpconnect/loader/mozJSModuleLoader.cpp#211-212

  fputs(utf8str.get(), stdout);
  fflush(stdout);