You can change the tabs and address bar positions (independently) in the Samsung Internet settings. You can also choose to display them always, on scroll, or never (for the tabs).
Umm, what? No, this is absolutely not accurate. They're laid out differently than desktop, but at least on android Firefox does have tabs. Not sure on iOS, but that's basically reskinned safari anyway.
I am writing this from Firefox on android. Neither phone nor tablet version has tabs. It has list of open websites ordered in a scrolling window as squares (and for sync reasons call them tabs), but no tabbar and no actual tabs. Chrome and Samsung internet has actual tabbar and tabs.
Are you using firefox focus? Because otherwise idk what to tell you. You can even see it in the images on this engadget post. In both images that they show...