Hey there, Firefox developer! Do you use Mercurial? Please take the time right now to run mach mercurial-setup from your Firefox clone.
It's been updated to ensure you are running a modern Mercurial version. More awesomely, it has support for a couple of new extensions to make you more productive. I think you'll like what you see.
mach mercurial-setup doesn't change your hgrc without confirmation. So it is safe to run to see what's available. You should consider running it periodically, say once a week or so. I wouldn't be surprised if we add a notification to mach to remind you to do this.