The Internet is a network with social effects. Whether "this didn't work" means "the website is broken" or "the browser is broken" has always been more about end-user experience and the wisdom of crowds than a more concrete definition.
A website broken only on Firefox works for 96.5% of users. I have personally had to make the hard judgment call (as a fan of Firefox!) to not spend 25% of our engineering debugging time on a problem only 3.5% of users encounter.