I guess this is the core of it. It is hard for me to understand why you'd think people working on a language would have "the primary product" be something other than the language. There isn't even a single coherent other project to be associated with. Why would "a browser" be special here? It’s even in the name: the Rust developers. We develop Rust.
Do you fault the C standards body for working on C? Or the Ruby core team for working on Ruby? What language do the developers of the language consider something other than the language as their primary product?
Now, of course, as a tool maker, you have to make sure the tool is fit for purpose. You want to enable the people who are working on projects like a browser to be able to do their job. But that's their job, not our job. It seems like you're suggesting that the primary job of Rust developers is to improve Firefox, and it's very confusing to me why you'd say that, when nobody would say that about any other language. Or at least, I've never come across that sentiment before.