Pedantry time: mach isn't a build tool, it's a command dispatch framework. So yes, |mach build| will build Firefox (or servo), but, for example, |mach try| will push code to the try server, and |mach mdn| will search MDN.

However to address the original point, Python is indeed widely used at Mozilla, not just for websites but for test harnesses, build tools, analysis scripts, etc. It wouldn't surprise me if it was second to only to javascript on the basis of "number of developers who have written >0 lines of code in this language for Mozilla projects".

Ha, thanks for pointing that out. You're obviously correct about it being a command dispatch framework. I've only ever used it as an interface to build and never realized what it actually did behind the scenes. And after reading a little more about it after you pointed out my mistake I have learned about an awesome tool.

I don't know how many people say this, but I appreciate the pedantry!