Rust was created at Mozilla to help with browser engine development, so it’s not totally shocking that it’s in Firefox.
Rust is in the Linux kernel in the sense that its build system can compile interoperable Rust code. But the kernel is big — it’s 30 years of C code written by thousands of contributors which receives dozens of patches (in C) per day. Obviously “Rust in the kernel” has to start somewhere, but as of today, it’s really only relevant to Rust people who evangelize it. A kernel developer who doesn’t read HN would probably have no idea that Rust support was added.
Going back to the OP, the claim that C++ is “becoming a legacy language” is…bold. IMO it’s a ridiculous comment until large C/C++ projects (e.g. Linux, LLVM, Postgres/SQLite, big AI/ML toolkits) are mostly rewritten in Rust or overtaken in market share by alternatives written in Rust. If all of the biggest systems software projects in the world are still mostly written in C/C++ and still receive almost all of their contributions in C/C++, claiming that Rust is bumping those languages to some “legacy” tier is ridiculous. Things might go that way in the future, but we definitely aren’t there yet.