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

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.