[pvg]

One reason is that Firefox is an older browser design (compared to Chrome), in parts retrofitted. That's a difficult, laborious rut to get out of, especially if your competition has more resources and a big head start. If you're starting a new browser variant, it doesn't make much sense to also start with Mozilla's disadvantage.

[shadowgovt]

Chrome was architected specifically to overcome the shortfalls of the IE and Firefox engines at the time. Firefox has not only been playing catch-up, it's been playing catch-up against specifically its greatest weaknesses.