Firefox is a full-featured browser, which is what most people want. Firefox Focus, however, is intentionally lacking in features with the aim of improving user privacy. That doesn't mean it needs to be completely barebones, though. With today's update to v2.0, Firefox Focus supports multiple tabs. Plus, it's a little more secure.
This browser is designed to store no data about your activities and block trackers/ads. At launch, it only supported a single web page at a time. Now, you can open multiple tabs but only for links in a page. Long-press on a link on your current page and tap "open link in new tab." All your open tabs are accessible from the floating action button in the corner, which is also where you can close all your tabs and delete history.
In addition to tabbed browsing, limited though it is, Firefox Focus supports incognito mode in Gboard. This doesn't appear to work on all phones, but it's functional on the Pixel. Maybe some sort of Oreo thing? At any rate, incognito in Gboard prevents the keyboard from saving anything you do. It's a perfect match for Firefox Focus.