Everything works the same, times of website incompatibility are long gone.

Not completely true. It's mostly true. I've daily driven Firefox for years, and the number of websites I've crossed that wouldn't function in it correctly but would work just fine in Chrome was very slim... but not zero. Definitely not comparable to the complete shitshow of the 90's and 00's. That's true. But it's not a completely solved problem.

And with Mozilla's leadership practically looking for footguns to play with combined with the threat of Google's sugar daddy checks drying up soon due to the antitrust suit (how utterly ironic that busting up the monopoly would actually harm the only competition...), that gap can get much worse in very little time if resources to keep full time devs paid disappear.