The only site I've found behaves terribly in Firefox is LinkedIn. Weird pauses on page load that lock the whole browser (not just the tab) for like 5-10 seconds. No idea what they're doing to make this happen, but it's odd.
Which is, well, fine, because LinkedIn is mostly a dumpster anyways.
This prompted me to check LinkedIn after months of having it parked in a tab and it worked with no problems.
When I have problems with a site it's usually because I'm blocking most JavaScript with uMatrix and I have to find the correct combination of scripts to make the site work for me without having to give away my soul to the gods of tracking.
As a software developer, it's been years since a customer told me that the sites I develop on Firefox don't work on Chrome or Safari. I don't even bother to check anymore. I couldn't check with Safari anyway and they are OK with that. The point is that if it works in Firefox it works everywhere. Of course we're not using any Chrome-only API but we never had to use one of them as far as I can remember.
Any site that is on the edge of performance (often due to bad engineering, which you can blame on time constraints) will perform vastly better in Chromium.
I don't much care, TBH. Having worked in the Chromium codebase before, I know what an absolutely mammoth amount of engineering hours goes into that.
V8 on its own is a technological miracle.
But all funded by a firehose of crazy privacy invading ad revenue.
So. I'll live with the odd pause and a bit of battery drain. I gave Google 10 years of my life as an employee in exchange for $$, I'm not interested in giving them the rest of my life for free.
It's funny because I worked in that repository for a few years, and routinely built a custom "shell" (what Chromecast is) and when this guy suggested Brave my thought was... eh, yeah, sure, but I could also just roll my own or just run stock chromium :-)
I personally like that Firefox isn't based on webkit/blink.
Okay, well, they're using React and most React engineers aren't very good so it ends up being a clusterfuck that most people don't even notice given how well Chromium is optimized.
Which is, well, fine, because LinkedIn is mostly a dumpster anyways.