It is 9.5% of the total code base, that’s even counting things like tests. I wouldn’t personally call that “very small” but ymmv.

That also said your parent equating memory safety to “no crashes” is also not correct. A process can exit early while never violating memory safety.

And even if Firefox was written in 100% Rust, Rust can still panic for logic errors. And system libraries have their own bugs and crashes that can bring down Firefox.