Try this analogy: Most people have functional legs, so why install a ramp? 99% of your users can access your property, so who cares, right?
People without functional legs can't simply decide to walk up some steps.
People use Firefox can simply decide to use Chrome.
To be more direct; what's your definition of "broken"? Is it that it doesn't work for you in the manner that you'd like?
Mozilla has had more time to work on this problem space than their competitors, and they don't have the technical advantage to show for it. They may have been the technologically better choice in the Browser Wars era of Internet Explorer, but nowadays? They're falling down on the technical merits, not just the network effects.
It's free and it's widely available. If they were better than the alternatives more people would switch to them but they're not.