AFAIR, the full build was also around 30 minutes on a desktop i7. Unfortunately, incremental build (even for the most tiny changes) was around 2-3 minutes which was a real killer for me when I tried to do some open source work on it.

There are tricks to make it faster. If you find yourself working on the project again, ask for advice on IRC. 2-3 minutes is longer than it should take in most cases. (As long as you aren't touching widely included headers files, that is.)

yup. 30ish for a full compile, less with ccache and other tricks. Rebuilds 2-3 minutes.

C and Rust? Firefox looks like a ton of C++ to me: https://github.com/mozilla/gecko-dev

Primarily C++, but Rust is there as well. However it's mostly libraries which were extracted from gecko itself, so it's dependencies, not main codebase.

I stand completely corrected, don’t know why I had that in my head.

The vast, vast majority of the Firefox codebase is C++.

You're thinking of Servo - https://servo.org/

Firefox now includes many rust components.