While I agree with some of your arguments, namely that "popularity and merit are not the same thing", I think saying that Mozilla is "trapped in a pseudo-C++ mindset" and that its new browser would ideally be written in Nim is plain wrong.
The fact that using Rust to write a simple website isn't very handy doesn't mean it's a bad language.
Web development simply isn't its primary focus: this is a systems programming language we're talking about. The mere fact that it is being considered for writing web apps is impressive since Rust is supposed to be a better C++, not a better Ruby, Python or Node.js.
Rust mainly emphasizes performance and safety. This means all the 'magic' that happens in a dynamic language is exposed to the programmer, and has a cost: the code is more verbose, and seemingly simple things are more complicated.
The idea that you can't get something good without a trade-off involving something bad is a false belief, an over-generalization that is very popular among people from all walks of life, including, unfortunately, technologists. Trade-offs do exist, but even a sword in fact has many more than two aspects.
The fact that using Rust to write a simple website isn't very handy doesn't mean it's a bad language. Web development simply isn't its primary focus: this is a systems programming language we're talking about. The mere fact that it is being considered for writing web apps is impressive since Rust is supposed to be a better C++, not a better Ruby, Python or Node.js. Rust mainly emphasizes performance and safety. This means all the 'magic' that happens in a dynamic language is exposed to the programmer, and has a cost: the code is more verbose, and seemingly simple things are more complicated.