People online rant about Firefox all the time for adding stuff Google and Microsoft shoved into their Chromium forks a few years ago, but when they do it the response is always "well what did you expect from <x>" while when Mozilla does it, the response is "this is an outrage, I'm switching to <some browser that already has the shitty feature anyway>".

I don't think there is or ever will be a "new internet explorer". If your page works in Chrome, there's a 99% chance it'll work in Firefox and Safari. Web standards have been unified to the point painting and layout algorithms are now part of the spec. It's why Ladybird managed to get a decently compatible engine in an extremely short time frame.