I use firefox exclusively, on both my laptop and my phone. It works perfectly on any website I throw at it. I work for a startup which makes video call apps, the web client works perfectly under Firefox, and there's a grand total of 2 devs working on it.
All this to say that if I come across your website and it doesn't work under Firefox, AFAIC it's your website that has issues, not Firefox.
As for the reason, you might be fine with a single megacorp dictating the way the web works, but for many of us who remember what it was like in the IE hegemony days it's a serious concern.