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 the number of tabs displayed in the dialog, including the current tab.
Possible values and their effects
The number of tabs to show as previews when switching between tabs in a window. Default value is 3.
First checked in
Has an effect in