Most common methodologies were already garbage even a few years ago, with general consensus being that Firefox was probably undercounted by at least 30%; and in more technical arenas, it can be a lot higher, perhaps up to 80%. (Unfortunately, that’s still probably not a lot.)

Even back then, Chrome was commonly being significantly overcounted due to user-agent string shenanigans. And these days I’m confident (without any figures, or even relevant recent commercial experience) that will have increased sharply. I expect that it is now massively overcounted, at the same time as Firefox is significantly undercounted.

Statcounter is particularly commonly used, and honestly one of the worst. Its mobile figures, for example, are completely useless because they don’t report browser versions. CanIUse figures (which lean heavily but not solely on Statcounter) are lousy and unrealistic due to some of these sorts of issues, and just generally being out of date. (I examined the matter closely on 2023-05-27 and the figures corresponded with being about six weeks behind.)