Hey, I'm on the chromium videostack team, would you mind filing a bug report for what you see happening with youtube? https://bugs.chromium.org/p/chromium/issues/entry
The annoying part is that hardware acceleration works on chrome OS, so we know the support is buried in there somewhere.
I doubt they will take Linux outside their own walled gardens seruously, Google has already shown their indifference to us Linux users.
I should add that for an identical Linux distro-and-version, and identical browser versions, my laptop (I forget the SKU, but Intel Broadwell U-series GPU) is about equal in performance between Chrome and Firefox. That is to say, 1440p60 runs smooth on each, so they're good enough that there's no issue from which to discern a difference (which is damned impressive for a 4-year old mobile chip, imo).