Firefox's font sizes in its settings page seem to be in pixels. This does not work well for a laptop which is sometimes used with its built-in screen which has s high pixel density, and sometimes used with a large external monitor with a much lower pixel density. If I set the font size to be comfortable on the external monitor, text is too small to read on the laptop's built-in screen. I can't carry a big (28") monitor with me every time I take my laptop out of my house (at least once a week). Of course I can switch the font sizes when I switch screens, but computers are supposed to be labour-saving devices, not work-creating devices.
It has been suggested to me in the support forum that I can change the meaning of the font sizes by setting layout.css.devPixelsPerPx on about:config, but I would still have to change that setting every time I switch screens. Incidentally the font size on the about:config page is small compared with the system font size, and hard to read, and doesn't seem to be controlled by the settings page. If Firefox is going to use its own sizes for its controls, it should allow the user to set them.
Most applications use the system font sizes, or allow users to set the font sizes in points, or allow users a choice of units. Setting font sizes in points should result in the text being the same physical size on each screen regardless of its pixel density. Please allow users to set font sizes in points for all text displayed by #Firefox, whether interpreted HTML or its own controls. Switching to points would break compatibility, so a practical solution would have to give users a choice of units