It's not a big deal since git repos aren't hard to migrate. GitHub is fine currently and if they push people away then there are a couple of alternatives.
Firefox hosting on Github is a good move because it lowers the barrier of entry for contributors.
Projects are more than just code. They are all the metadata, ecosystem, and people around it. You can easily move a git repo, but try moving github issues or github PRs, pipelines, community questions, and so on. You'll realise how much of a fallacy "It’s not a big deal since git repos aren’t hard to migrate" is.
Lowering the barrier to entry by moving from a technology few use (mercurial) to something popular (git) makes sense. Requiring participation on a proprietary platform owned by Microsoft instead of an open one like Codeberg or GitLab is just lazy. If someone wants to contribute to Firefox, asking them to create an account is a small ask, and I'd argue that if they're unwilling to do even that, then their participation in the community is likely to be far from useful.
Indeed. This "GitHub is owned by Microsoft, therefore evil lurks around every corner" thing has been going for many years now with no sign of the promised apocalypse and no real reason to expect said apocalypse. Back in the day I used to do a lot of modding for an open-source game and almost all the mods were hosted on GitHub, but then when Microsoft bought it about half of the modders threw an ideological fit and moved their mods to a wide scattering of other hosts. It made everything so much more of a hassle to fork and submit issues and whatnot, I'm sure it's done more harm to the project than anything Microsoft would ever do.
500M probably could buy that, but what would that cost look like in comparison with using GitHub? When you are on a fixed income, making decisions based purely on philosophy becomes a luxury.
They could have opted for Codeberg for example and made a public donation to the project of a few hundred dollars a month. Instead, they opted for funnelling more power and support into a terrible company.
Well, going with Github wasn't quite as stupid a decision when they made it, which iirc was so long ago that it hadn't yet been acquired by Microsoft. As with other typical corporate problems, once they got started down that road and had some sunk costs they couldn't find the strength to turn back even long after it became apparent that it was a mistake.
Bugzilla is still where they are managing bug reports and contributions will still go through (I think) phabricator. Note the lack of Issues and Pull Request tabs on the GitHub repo. This is more just a change of hosting than anything.