Video viewing (as opposed to previewing) really belongs outside the browser anyway.
Using mps-youtube to search, curate, manage, and play lists of videos, without relying on YouTube's server-side management is a real bonus.
Hooking mpv into the stream more smoothly would be useful, and I don't have a good method yet. But given my primary use-case (listening, audio-only), the fact that in the case of video and audio you really can only watch/listen to a single performance at a time, and the fact that in general what's most useful for me is to have a queue of content that I'm going to watch/listen to, in sequence, non-browser tools turn out to be far more useful than browser-based ones.
Most especially when you throw auto-play and advertising into the mix, and find yourself with multiple auto-playing tabs amongst ... well, I'm embarassed to say just how many browser tabs I typically have open, but it's a lot.
WFM. YMMV.
If my split was different I'd feel quite different about it... I just need Hangouts, Mapbox, YouTube, etc to work 100% of the time without headache. Also running two browsers was difficult because it's like "where's that darn tab!!!" half the time ha.
My solution to this crap is to eventually do something like a pihole setup upstream from the browser and call it a day.