Tracker/analytics data won't be correct for Firefox or people using privacy-centric browsers as they inherently block that. I think it'd be hard to get a real number without polling a lot of people.

A lot of people assume Chrome has the monopoly based on looking at false analytics.

Firefox definitely sends its user-agent to servers, so wouldn't server-side analytics still work?

Theoretically, server-side analytics could use the user-agent strings and other headers in the request to have infallible analytics. In practice, a lot of server-side analytics in practice rely on third-party tracking scripts (like Google Analytics) and other such practices that get blocked by tools like Privacy Badger, etc.

Perhaps, and this is likely the case with most users. But there still exists many users with extensions disabling fingerprinting and such.

I wish Cloudflare would publish stats based on user agents. They must have a good idea about the real numbers

What do you want to know?

What percentage of traffic comes from Firefox.

It looks to be between 60-70% depending on what data is used.

https://www.w3counter.com/globalstats.php