On Android, with the OS set to use the bottom 3 buttons (back, home, app switcher) and Firefox set to use bottom address bar, when multiple tabs are open, closing a tab causes the "Tab Closed" bar to cover the lowest tab in the list. This causes problems when trying to close multiple tabs because it's no longer possible to hit the "X" marking on the next tab without swiping the box away first. I don't recall exactly then this became a problem, but it was fine until a few months ago and then a change caused this issue to start. There's a lot of empty space in the box, maybe smush the box to the left side of the screen so the next tab is still able to be closed?