Adblock on a mobile browser is a gamechanger. I wouldn't go back even if a quarter of sites were broken on FF mobile (and they aren't, I've been using it for 2 full years desktop and mobile, and only found a single compatibility issue - a restaurant's take-out ordering page)
Honestly, in this day and age, any discrepancy from Chromium/WebKit is probably broken. But if we're talking about a few pixels' difference here and there, plenty of sites are more broken than that on every browser.
I have encountered a couple of (financial, natch) sites that insist on a Chromium-based browser in order to do vital operations like download statements. Switching browsers for those few exceptions is annoying, but not a deal-breaker (for me).
Have you tried a user-agent switcher? I think you can even set them up to always have a specific agent depending on the domain. Chances are, they don’t need chromium, they are just written by (even more than you’d think) incompetent people.
It could be that my baseline was the Chrome rendering, so any discrepancy was classified as "it's broken".