>But this is a technical document about browser features, I'm a little frustrated that this is being coordinated over a proprietary SaaS service instead of over Git[0] with markdown or something.

Then maybe the public SaaS services should be of a better quality so that Mozilla employees want to use them.

>I think a lot more than that, I'm frustrated that the document is being made publicly available on that platform. When someone links to a web standard on Github, often it's because the standard is seeing active participation. For something that is being linked to a public view, I would have loved to see at least a PDF export or something.

There is a PDF export for google docs which is available to all users. File > Download > PDF.