This changes Firefox's behavior of switching tabs for Ctrl(Command)-Tab, Ctrl(Command)-Shift-Tab, Ctrl(Command)-PageDown and Ctrl(Command)-PageUp. By default, Firefox switches to the next or previous tab when you press those keys. With this extension, Firefox only shows the list of tabs when keys are pressed and switches to the selected tab after you release keys.
By default, this extension provides behavior about tab switching like Alt-Tab on MS-Windows. Tabs are listed like stacked cards, and the last selected tab will be listed on the top. (Of course, you can change the listing algorithm to a simple list.)