This seems apt: https://www.joelonsoftware.com/2000/04/06/things-you-should-... Mozilla spending time on this suggests Mozilla doesn't know what to spend time on.
Ironically Mozilla is the best counter-example to that Joel on Software article. The article specifically mentions that Netscape shouldn't try and re-write their browser. But if they hadn't done that, we wouldn't have Firefox. We'd still have an incremental improvement over Netscape 4.7. For those who can't remember Netscape 4.7, let me remind you, it wasn't very good at the end. If they hadn't rewritten it, causing…
I've been in a talk of a mozilla developer like a decade or more ago, shortly after they had released the rewrite. His comment on starting a project like that from scratch: "don't". What they intend to do (slowly and gradually transition to Servo) seems consistent with an organization that has learned from this experience.