I can make a clean build of Firefox in less than 15 minutes on my MacBook Air M2, C++, Rust, and all. If you’re working on a frontend feature that only needs to modify Firefox’s frontend JS code, you can use “artifact builds” (prebuilt object files) so you don’t need to recompile the native code.
Wow, your rust experience is way different from mine (not just FF, I mean invocations of rustc are where time goes to die). I also readily admit that I struggled constantly with trying to get sccache to behave sanely, so I'm open to that being part of the difference, too