[Xylakant]

I don't see why this is a reason to be skeptical. Firefox is a huge codebase. Substantial chunks of it have been rewritten using Rust while maintaining the entire app and adding features. This is entirely the plan: You can slowly, as you touch parts, migrate the codebase.

Firefox (and many other projects) was already a mix of multiple languages - it uses C, C++, Javascript, ... and now Rust. And while there is value in having less languages to deal with, there is equally value to use multiple languages and apply them where you see major benefit. For example, replacing exposed and high-risk components (image/file parsers for example) with a language that de-risks those implementations. Like all of the things in software engineering, this is a tradeoff. It may not make sense for your application, but it can make sense for others.