Comment by quickslowdown

For me personally, it comes down to diversity. If all other browser engines "go under" and we are only left with Chromium, websites can only be built for 1 browser. Although Chromium is a great engine (evidenced by how many browsers are built on top of it and how widely the underlying Chromium engine has been embraced), it's not without quirks, bugs, flaws, and vulnerabilities.

Firefox is built from the ground up in a different way from Chromium, with its own set of bugs, quirks, flaws, and vulnerabilities. There may be some overlap, but having entirely different architectures means we keep pushing the compatibility envelope, we get "copycat" features, where one engine does something great and the other implements it in a way that works with their own engine, etc.

It's just better to have more than 1 browser engine around. I wish it wasn't so difficult to start a new engine from scratch today, the sheer amount of features a web browser must have to get people to even consider reading your About page, and the overwhelming complexity of modern webstacks, mean you basically have to be grandfathered in as a browser that's been around for decades and has a huge amount of community support.

I am aware of alternative browsers, Arc and the like. I'm very happy to see someone seriously go after an entirely new browser engine that's not Gecko or Chromium, and the traction they're gaining while not being "fully featured" sort of sums up the sentiment of my message (I hope). Having alternatives is good.