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They are not on the fence about Rust. They are on the fence about JPEG-XL when it entails a major increase in attack surface due to the current implementation being in C++.

Or Ada / SPARK which is widely used in production for critical-systems.

Rust adds a lot of complexity (a new language) and does not even offer a general approach to automated correctness verification (not saying that C++ is better in this regard). Maybe it is just better to wait for Rust++.

Rust is much better suited to static analysis and formal verification than C++, primarily thanks to stricter pointer aliasing, immutability without loopholes of const, and thread safety annotations in the type system.

Most things that are advanced correctness verification in C++ are regular compiler errors in Rust.

Rust is already in the firefox codebase (hell, it was basically invented to be used in firefox) so it doesn't add any complexity.

Rust is not a new language for Firefox, some of Firefox is already written in Rust.