It would be great to create the html/css/javascript stack from scratch, or at least make a non-backwards-compatible version that is simpler and can perform better. HTML5 showed us that can work.
The whole Mozilla strategy of corroding Firefox piece by piece is actually very professional. Big backwards-incompatible transitions in technology almost always fail.
FWIW this is usually due to folks doing performance work in only one browser or not really testing well and slapping that label on after the fact.
Or stuff like Hangouts and Allo where they use nonstandard features.
The major things Firefox doesn't support that Chrome does are U2F (it does support it now, but flipped off, will flip on soon I think) and web components (support should be available next year I guess; this kinda stalled because of lots of spec churn and Google implementing an earlier incompatible spec early or something.)