I do not remember the last time a website actually failed to work in Firefox. Once in a blue moon I'll come across a site (looking at you, SnapChat) that claims it won't work, but suddenly is just just hunky-dory with a quick User-Agent string swap. We've gotten to a point where if your site doesn't work identically in Firefox and Chrome, you're probably doing something pathological.

With regards to APIs, I'm very happy with Firefox's commitment to telling Google to go fuck itself when it comes to Manifest V3 neutering of adblocking.