They're switching their main repository from Mercurial to Git. Mozilla started using Mercurial before Git became de facto standard, but I imagine these days learning Mercurial is seen as an unnecessary obstacle for new contributors, hence the current switch.

As for why GitHub specifically, it's because that's where the rest of Mozilla's projects already are. They have been using GitHub for a long time (14 years or more), with thousands of repositories there. It's why Rust and Servo are on GitHub, for example.

Edit: See https://glandium.org/blog/?p=4346 for more thorough/accurate info.