The absolute number is not that important, the trend is. That's clear even in Mozilla's own data: https://data.firefox.com/dashboard/user-activity shows they went from 303M monthly active users at the end of Nov. 2017 to 277M this year. This is a serious drop.
Looking at netmarkshet website and web requests it seems this pulls from gator.io for metrics. Looks like 3k-4k websites have this data which lines up with the previous statment.
https://publicwww.com/websites/%22gator.io%22/
