[davidhaymond]

It's called Responsive Design Mode, and it's the third button from the the right on the top bar. Even better, use the shortcut Ctrl-Shift-M, which works even when devtools is closed.

[cvladan]

Chrome is inferior here in this area.

Because you can activate mobile view in Firefox WITHOUT devtools opened, but in Chrome that's impossible.

Lot of times I hated this at Chrome.

Also, that shortcut in Firefox will immediately give you mobile view, and in Chrome not as you first need to type shortcut for Devtools.