It's looking at the width, so that some fixed-width elements don't run into each other and wreck the display - if you expand the window a little it should work as expected.
For what it's worth, I'm also running Firefox on MacOS, so I'd expect that to work given the right window size.
I went to the site on my 27" iMac and got the 'Desktop Required' message.
Resolution per Panopticlick [0] is 2560x1440x24.[0] https://panopticlick.eff.org/