From MozillaZine Knowledge Base
Firefox 3.1 offers previews when switching between tabs, showing page titles and thumbnails of content to the user in a pop-up dialog. This preference determines whether the switching between previews uses a smooth-scrolling transition.
Possible values and their effects
Scroll tab previews smoothly each time Ctrl+Tab is pressed. (Default)
Instantly switch tab previews when Ctrl+Tab is pressed.
First checked in
Has an effect in