Disabling Touch Simulation in Firefox Renders HTML/CSS Differently

You need to add following code to the head section (before title tag):

<meta name="viewport" content="width=device-width, initial-scale=1">

From HEAD

• meta name="viewport" – viewport settings related to mobile responsiveness
• width=device-width means that it will use the physical width of the device (instead of zooming out) which is good with mobile friendly pages
• initial-scale=1 is the initial zoom, 1 means no zoom

For more info about viewport tag see: Using the viewport meta tag to control layout on mobile browsers