Yeah, that's true. But...if a website derives its revenue from ads, a non-ad-viewing user
and users who switch browsers so as to use an ad blocker would presumably be blocking ads
loses that website money rather than generating it. Like, for such a website, the issue would be how many Firefox users that do view ads would be lost if their website didn't work on Firefox.
Those of you who remember the early 2000s, when Internet Explorer had very high marketshare, probably remember a number of websites that didn't work with other browsers.