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.
I use Firefox across all my devices. It's fantastic. The people who actually care about browsers (and, perhaps, "anonymity", or "doing the right thing") have all largely moved on from Chrome.
Okay, that's good. Hopefully they will continue. Goal is to not have single-browser monopoly, so everyone develop according to the standards not optimize against single engine.
What's the point in arguing who is better balance keeper?
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.