To give a bit of additional context here, since the link doesn't have any:
The Firefox code has indeed recently moved from having its canonical home on mercurial at hg.mozilla.org to GitHub. This only affects the code; bugzilla is still being used for issue tracking, phabricator for code review and landing, and our taskcluster system for CI.
In the short term the mercurial servers still exist, and are synced from GitHub. That allows automated systems to transfer to the git backend over time rather than all at once. Mercurial is also still being used for the "try" repository (where you push to run CI on WIP patches), although it's increasingly behind an abstraction layer; that will also migrate later.
For people familiar with the old repos, "mozilla-central" is mapped onto the more standard branch name "main", and "autoland" is a branch called "autoland".
It's also true that it's been possible to contribute to Firefox exclusively using git for a long time, although you had to install the "git cinnabar" extension. The choice between the learning hg and using git+extension was a it of an impediment for many new contributors, who most often knew git and not mercurial. Now that choice is no longer necessary. Glandium, who wrote git cinnabar, wrote extensively at the time this migration was first announced about the history of VCS at Mozilla, and gave a little more context on the reasons for the migration [1].
So in the short term the differences from the point of view of contributors are minimal: using stock git is now the default and expected workflow, but apart from that not much else has changed. There may or may not eventually be support for GitHub-based workflows (i.e. PRs) but that is explicitly not part of this change.
On the backend, once the migration is complete, Mozilla will spend less time hosting its own VCS infrastructure, which turns out to be a significant challenge at the scale, performance and availability needed for such a large project.