Programming Humor

That's still only testing one browser, unfortunately, and especially for mobile, Firefox is a very small percentage of users

You ideally need to test Chrome, Safari & Firefox as the bare minimum, at least the latest Samsung browser is a good idea too as it's often based on an old version of chrome. Thankfully IE has dropped off the list for most engineers now.

It allows testing on different screen sizes and orientations that match certain devices (and allows you to input custom sizes). It also gives you the option to set the User Agent string so certain styles that might be programmed into the site will trigger. However, it does not run the code in different engines. It's still helpful, but will not show all the bugs or performance issues. For example, the way they render SVG images is slightly different. A certain image that loads quickly in chrome could potentially take longer in Firefox.

But to your point, you don't need all the devices in the above screenshot to do testing. If you really want to do it manually, you can do so with just Firefox, Chrome, and Safari and use their respective emulators to vary the screen size.