How to force firefox to show mobile version of websites?

Firefox

Showing the mobile version of websites is a built-in feature in Firefox. In Firefox select Tools -> Web Developer -> Responsive Design Mode and then from the dropdown menu select your device model.

Chromium/Chrome

Toggle the Device Mode button to turn Device Mode on or off. When Device Mode is on, the icon is blue (). When Device Mode is off, the icon is gray (). Select Tools -> Developer tools -> click Toogle device toolbar button (marked by the mouse cursor in the below screenshot -> click the black downward triangle ▾ to the right of "Responsive" to show the dropdown menu and select your device from the dropdown menu.

You may use an add-on like User-Agent Switcher.

It lets you choose from many options:

(screenshot source: link above)