Earlier quoted context omitted.
From a tech geek perspective, Firefox is the "emacs of browsers". They wrote a (lisp/js) interpreter and then wrote the (editor/browser) in that language. Unfortunately using a much more weakly-typed language [to write what is almost an OS of its own] yielded a mass of unmaintainable, constantly-mutating bloatware. They claim to be rewriting the interpreter layer in Rust, which is awesome, but 99% of the browser will…
Not sure where you're getting your information, but the majority of Firefox is written in C++, not Javascript.
The vast, vast majority of that C++ is legacy crud that gets #ifdef'ed out -- Mozilla development style seems to be to never remove code, ever; just #ifdef it out. Also there's a ton of boilerplate in there due to their homegrown IDL.