For a long time Firefox Desktop development has supported both Mercurial and Git users. This dual SCM requirement places a significant burden on teams which are already stretched thin in parts. We have made the decision to move Firefox development to Git.
We will continue to use Bugzilla, moz-phab, Phabricator, and Lando
Although we'll be hosting the repository on GitHub, our contribution workflow will remain unchanged and we will not be accepting Pull Requests at this time
We're still working through the planning stages, but we're expecting at least six months before the migration begins
You have got that backwards. Github became popular because git was popular. Nobody forced people to use it and which VCS the Linux kernel uses barely matters to anyone not working on it.
The truth is that a "nice" UI that doesn't allow you to know what actually happens because the internals are hard to understand is hard to trust with your code history that you still want to access 20 years later. Git is popular for the same reason Markdown, Textile,... are popular among programmers over WYSIWIG style rich text programs like Word.
Nothing in tech stands still. If you want a glimpse of a possible alternative future check out Pijul. And I don't know an example off-hand but the idea of doing version control on ASTs of program code rather than flat text is an interesting concept that hasn't taken off yet.
Git seems to have reached a status of "good enough" to the point where it will probably stand still for quite a while. There just isn't anything that offers benefit significant enough to justify the bother of switching (that I've seen so far, I've seen pijul before).
Plenty of things do stand still. We're still stuck on qwerty keyboards for english.
There are very good reasons why Git prevailed. The previous generation of SCMs were terrible. Most of the ones that came around the same time as Git (distributed SCMs) got lots of things wrong. Git is complex but it's flexible, it doesn't try to force any particular way of doing things on you, which means you can make it work with any flow you can think of.
Just find yourself an antiquated Microsoft aligned company and maybe you'll get to taste the bitter fruit that is TFVC. I use it daily and it's.. Different.