I think that actually speaks to Joel's point. None of those were rewrites.

Firefox started as a fresh UI reskin for the old browser interface, and indeed continued as "mostly a skin" for years (incidentally, so did Chrome).

(You can still get the non-firefox skin incidentally, it's "Mozilla Seamonkey")

Then the Rust rewrite. Rust was a hobby project, and they rewrote layout engine interface, and nothing more than that. Then CSS.

Now it's an HTML parser, a layout engine, a parallel CSS engine, and a GPU webpage renderer (still "missing"/non-rust are 2d compositing and javascript). Each of those components replaced another in a working version and there were at least beta versions of firefox of all the parts.

You don't know what we would have instead.

Potential is worthless. We have Rust, we don't have what might have been produced had Rust not happened, and as far as I know, no one is working on that hypothetical product.