Firefox development is making a big change from using Mercurial to Git. The decision was not mine, but as someone involved in the process, I feel partially responsible. Let’s take a step back and look at the history leading up to this decision. In the early days, the Mozilla source code was stored in a CVS repository, then moved to Subversion. Around the same time, Git and Mercurial were born as distributed version control systems. Ultimately, Mozilla chose Mercurial due to better Windows support. Over the years, Git gained popularity and Mozilla engineers started using their own Git mirrors. Now, with the switch to Git, Mozilla is embracing the growing Git mindshare.
https://glandium.org/blog/?p=4346