We know. One of those things that was established 10+ years ago and has a lot of built-up inertia keeping it going.
It's part of my job role at Mozilla to bring this process into modernity. We're getting there. Slowly. GitHub pull requests will be supported in some form. Hopefully by July.
Like git, Linux, Mercurial, and gcc development, they have a patch-based code review flow, but MQ is not needed to produce the patches that they expect. You can do the same without MQ, and they will never know the difference on their end.
I actually rather like email-based code review. It's really easy to keep editing and rebasing your patch while people comment on it. If Mercurial Evolve picks up, this will be an awesome new way to work, especially if Kallithea or Bitbucket grows the features to support it.
However, current Mozilla development, AIUI, is moving towards ReviewBoard, although I have heard a growing mutinous group in Mozilla who just wants to chuck all homegrown solutions and slap all of their code on github like everyone else.
But if you are used to Github/PRs their Bugzilla/Hg-Patches workflow feels like going back in time... way back to the old days.