Small screen menu is not responsive on smartphone

Hello,
my experience is that when I resize a Firefox window on my desktop PC to be as narrow as a small smartphone screen the menu behaves nice responsive as expected – after clicking the “small screen menu icon” it expands into a properly sized menu column shifting the page content to the right (preserving size). However, on a real small screen Android smartphone it behaves different: after touching the menu icon the page (including the “small screen menu icon”) will get shrunk (downsized) and the menu column expands shrunk as well, too small to read. It can then all be magnified by the standard two-finger touch-stroke. This is, however, an undesirable non-responsive behavior. What can we do?