That's why Rust was introduced into Firefox piece by piece. The goal wasn't to rewrite firefox in Rust - just to migrate the scary bits over to a memory safe lang. You can feel a lot of that in the servo codebase, weird pointer semantics as a result of needing to be API compatible with the C++ adapters.

If I were building a company around a new browser, I'd reach for the solid stuff that can be pulled in. Our whole blitz project is designed to be modular exactly for that use-case.