This changes Mozilla's behavior of switching tabs for Ctrl(Command)-Tab, Ctrl(Command)-Shift-Tab, Ctrl(Command)-PageDown and Ctrl(Command)-PageUp. By default, Mozilla switches to the next or previous tab when you press those keys. With this extension, Mozilla 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.)
If you mostly use the mouse device than the keyboard, Joe4711's TabScroller will be more helpful than this.
If you use Mozilla Suite or Netscape 7, see How to install and update? (contains important notes) and How to uninstall by hand?