This thread is becoming like every previous one where we talked about web browsers and that is finding the BEST web browser. Let me tell you something immediately—there isn't the best one! Every single web browser excels in different area.
No one is denying that out of all browser engines, Chromium (Blink) supports the most web standards; this is also what makes it fast. The only reason why they could do this is because they have HUGE team behind the project and because Google practically controls entire internet. If Google wasn't behind the Chromium, I guarantee, its web standard support would be on the level of Mozilla's Gecko or even Opera's ex. Presto. So the only reason why Chromium is so good at compatibility is Google which monopoly literally develops and mandates which web standards will be used and which wouldn't.
Now... when choosing your web browser of choice, you can't only look at support for web standards, you have to look at other areas as well. And this is where competition (the only real competition), Mozilla shines. We can argue as much as we want, but the truth is there isn't a single web browser on the market that is customizable to the extend Mozilla's Firefox is. Yes, Firefox even surpasses Vivaldi in that question. And I'm not even talking about UI customization (that is completely possible on Firefox); I'm talking about how the browser itself works and functions. This is something you simply don't get with Chrome or any other Chromium browser. What ever you want Firefox to do, it simply can by tweaking about:config settings.
For me personally, the thing that matters the most to me when choosing a web browser is ad blocking, privacy and control. Support for web standards is still high on the list, but definitely not on the level of those three. Why? 99% of the websites use web standards that are completely supported by major web browsers. The 1% is those benchmark sites that use experimental web standards still in development. Google is usually developer of those so it makes sense that Chromium would be the first to implement them despite not being finished. Mozilla prefers to implement new web standards once they become finished and widespread as there's not really a reason to implement them before when they could focus workforce to more important things. Mozilla doesn't have as much as workforce as Google does so they have to focus on a few things at a time instead of 1.000.
After using Firefox for close than a year now, I still haven't found a website that doesn't work in it. And even if it loaded websites 20ms slower, I rather take that then reduced ad blocking functionality and sacrifice privacy.