[Sylos]

Trust me, if Mozilla actually had a choice in the matter, they would opt for just having it all in Rust, too.

But there is no choice. Rewriting Firefox from scratch is going to take decades. Firefox has to continue to function in the meantime. They do occasionally replace components with equivalent Rust components from Servo, and that's so far been a great success, but it's still scary as all heck to take a software that millions of people depend on in their daily life and wholesale replacing the CSS engine, URL parser or media decoder in it.

Besides that, it's not like Chrome/Opera, IE/Edge or Safari are bastions of security. Users can't go anywhere that's decisively safer.