Less than 40% use extensions. Of the most popular - uBO is less than 5%, so most people likely aren’t blocking it unless it’s the default.
That may be true that dashboards are under-counting Firefox, but the ultimate impact is the same. Without some kind of evidence that more than a trivial number of hits come from Firefox why would any small business pour time into catering to an audience that might exist instead of pursuing one that does?
Firefox sends an easily identifiable user-agent string with every request.
If businesses want to operate on bad data (analytics that depend on JS), that's their fault.
From Adobe analytics and sometimes Google analytics.
You're right that ad blockers are on the rise, but that's also true for Chrome.
Note that the points I made regarding market share seem to be consistent across the board. Whether you check Statcounter, an alternative to that, or the analytics of large websites. The story is the same: Firefox is tiny, lost in a see of dozens of equally tiny browsers.
Where are your dashboards getting the data from?
I use Firefox, but like many Firefox users I use uBlockOrigin so I block requests to trackers like Google Analytics and StatCounter.