Firefox is great as a product and even greater for this reason. All other browsers are either packaged with the OS (by the company selling the OS) or as you said.
Firefox accounts for about 5% of browsers used. It's not dead, but far from thriving, and many web sites ignore Firefox to the point they are not compatible with it. Yet I have not seen a major impact of this quasi-disappearance.
True but until very recently there was also Microsoft in the game.