Once upon a time, a very common feature in custom Android ROMs was the ability to double-tap the app switcher button in the navigation bar to quickly switch back to the previously active app, akin to Alt-Tab on many desktop operating systems.
I would like to see this same gesture added to the Tab Manager button on Firefox for Android. Toggling between two open tabs is an incredibly common use case for me, and they are frequently not right next to each other in the tab list, making swiping between them cumbersome. I see this working exactly like Ctrl-Tab does on desktop (when the "Ctrl+Tab cycles through tabs in recently used order" setting is enabled).
Bonus points: It would be great if this feature worked with private/tab group tabs as well!