It’s a nice feature for those that actively enable it and know that it’s enabled, but not for the average user. Most people never change the default settings. Firefox breaking stuff by default would only decrease their market share even further. And this breaks so much stuff. Weird stuff. The average user wants a browser that “just works” and would simply just switch back to Chrome if their favourite website didn’t work as expected after installing Firefox. Chrome can be used by people who don’t even know what a browser is.