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...
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.
On the bright side, it keeps a running diff of the css changes you make. Super useful when you're trying to fix some stupid layout thing, get it working and then forget all the changes you made.