It would actually be a very interesting experiment to try running Servo Emscriptenified (with SharedArrayBuffer, of course). I wouldn't want to write a browser in the full JS language, but that's precisely what asm.js is for.
"You should abandon Rust and write in JS" presupposes exactly the kind of strawman I've been arguing against. You don't have to abandon your favorite language!
This is a very strange comment, especially coming from you.
Everybody knows that the bulk of Firefox is not written in JS. You personally have spent years of your life working on a new programming language precisely because JS is not suitable for writing a web browser.
I'll take your position on JS seriously when you abandon Rust and begin writing the bulk of Firefox in JS.