Discourse composer and Firefox on Android tablet

Can we fix the problem that the content in Discourse becomes that large when I enable desktop site? I guess not, because that doesn’t seem to be a Discourse problem, because it happens at other websites too. But the increased size of everything is why I don’t want to enable desktop site.

Desktop site disabled:

Desktop site enabled:

Note: the full-screen composer button is also missing when desktop site is enabled and in Chrome.

How does Discourse decide whether I see the mobile composer with the buttons at the bottom right or the desktop composer with a preview next to it? Since I see the desktop version of the preview the detection for that seems to work in Firefox. Can the same technique be used for the option to change the height?