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).
Im a decade+ daily driver of Firefox. If a website doesnt support it, I make a mental note that I hate this company now, close the tab, and move on. I don't open it in a V8 browser, i look for an alternative.
I've gone back and forth between chrome and firefox for the what has to be close to 20 years. I want to use Firefox but ever since Chrome came out and blew it out of the water with javascript performance, it's always felt a bit clunky even as they've made their own performance improvements. I suspect my future lies with a chromium based browser eventually.
I think this is wrong. It starts with a website to look up train departures that no longer works on Ff. Then the site to buy movie or concert tickets fails on FF. Booking a restaurant, again, requires you to take out chrome.
Which has two concequences. People stop using FF all together because switching all the time is work. And people ensure they always have a copy of chrome around, while fewer and fewer need to keep a copy of FF around.
And so FF spirals exponentially into insignificance.