It doesn’t mean much, it’s just a metric people like around here. This number can grow and shrink just as easily with spoofed user agents strings. I think brave spoofs it and there’s a chrome extension, there maybe a few more examples.
I wouldn’t take it at face value is what I’m getting at. There’s just no other way to measure because most distros don’t collect telemetry and Firefox doesn’t seem to make theirs public.