Git did not run on Windows when Mozilla moved. Git on Windows required running Cygwin was prone to breaking on Cygwin and was slow. This was a pretty big dealbreaker for Mozilla. The imports of Mozilla's ~10 years of CVS dev via the version control's import tool was fragile and none of them imported cleanly without patches.

https://soberbuildengineer.com/blog/2007/04/version-control-...

https://soberbuildengineer.com/blog/2006/11/version-control-...