Till now Firefox for Android in Preview (which is being called Fenix by Mozilla) shows tabs open in the browser on the home screen itself, this to change with the new Tab Tray feature developed by the company.
Firefox for Android stable will soon be Fenix, which is the codename internally assigned by Mozilla for Fennec successor. As of writing today, Firefox Nightly and Beta are already using Firefox Preview browser engine, Geckoview, and supports Web Extensions. Mozilla altogether offering six recommended add-ons in Nightly version of Firefox.
Mozilla is developing Tab tray feature where if you click on Tab Switcher icon it now displays open tabs with a bar at the bottom with Private browsing, Home, and New Tab icons. Currently, Firefox Preview shows tabs opened on the Home screen and allows to close or share them or save to a collection from the three-dot icon.
With the latest developments taking place, the open tabs section has moved to a new location and the Home screen now displays top sites and collections only, this may change in the future. You can use the search bar to visit a website or tap on the tab switcher icon (which also shows tab count) to see a list of recent tabs where you can utilize the + icon at the bottom to open a new tab.
Source: Firefox Preview.