Given the most popular desktop resolution reported, I'd say "not most".

It's always interesting to see what the rest of the world is doing outside of the bubble.

4K display (3840x2160) with @2 scaling is being reported as 1920x1080:

    gfx.monitors.[0].screenWidth	1920
    gfx.monitors.[0].screenHeight	1080

So the resolutions do not say anything about HiDPI. There's an additional key for that:

    gfx.monitors.[0].scale	2

Zooming (ctrl +,ctrl -) in Firefox is terrible, and with a 4k screen zooming is a key feature. I quit using firefox because of that. Chrome's zoom actually works.

I just changed the devpixelsperpx preference in Firefox. No need to mess with zooming anymore. FF should read the screen density automatically, but as annoyances go, this is a minor one (just set and forget).

It actually used to do it automatically, but it got more complaints from people who were surprised that it did so than complements from people who were thankful that they could read things without squinting.

Give it time, it's only been nine years... https://bugzilla.mozilla.org/show_bug.cgi?id=400364