> Imagine if you have a website with 1,000,000 monthly visitors, and 2% of them use Firefox. Dropping support will mean 20,000 less monthly visitors.

It will always be 2% which will always be, at maximum, 2% more revenue. That's probably negligible if you have to support them and there's an opportunity cost to that money. You'd probably be better off spending that money on advertising than Firefox support, especially since Firefox users typically know when a page isn't working, it's probably them and they have a backup solution available.

I used Firefox for a long time but switched because I just got tired of switching all the time and I started regularly using a site that didn't work in Firefox (I don't remember now but I think it was my credit card).