> So my first one is 'Chrome devtools has a little button to instantly switch to mobile view, where you simulate (more or less) the viewport of various mobile devices'. I don't see this in Firefox.

The icon is slightly different and is in a different place (same vicinity though, somewhere on the top-right), but the "Responsive Design Mode" is equivalent to Chrome's "Device Toolbar". See https://firefox-source-docs.mozilla.org/devtools-user/respon...