Which shared a lot of roots with Chrome. It's probably that they are optimizing for things that are fast in Chrome's JavaScript engine, which isn't necessarily what's fast in Firefox's Javascript engine.
There's a decade of history in these engines with a lot of man hours put into them, and they were actively competing with each other very loudly for a while. There's little you could tell me I couldn't find some possible narrative justification for if I consider the PR, bureaucracy, talent and design decisions that might stem from those over a decade.