> Large parts of web browsers (like entire Firefox' UI) is written in javascript already

Is UI even a large part of Firefox? I imagine that the rendering engine, JS engine, networking, etc, are many times larger than UI.

Most programs should be written in GCd languages, but not this.

Except in a few cases, GCs introduce small stop-the-world pauses. Even at 15ms pauses, it'd still be very noticeable.

There are GC algorithms that don’t require stopping the world.