To be fair, you are correct in the mobile space, and that is now the lion share of devices, but Firefox does have a healthy 7.5% usage in the desktop space if the metrics on this page are to be believed.
It affects usage rates of other browsers other than Safari on that platform, because other "browsers" are just limited chrome on top of Safari. The quality of Chrome, Firefox, etc on iOS will be behind the quality of the same browsers on platforms that they're allowed to improve and innovate upon.