That here is kind of the problem.
The Firefox DevTools aren't meant to simulate the whole browser experience you get on a mobile device. It is only meant to emulate the sizes and to a certain degree the touch functionality.
For example, nearly any mobile phone will just pop up a separate list, if you select any type of combobox. But if you use DevTools, you won't get that 'popup'. It will only do the drop-down of the list.
Just go to any website you know, that has a combobox and try it there. It won't work right with the DevTolls.