From MozillaZine Knowledge Base
This preference is removed in Firefox 31. An Add-on restoring its functionality is Classic Theme Restorer. An add-on restoring all the close button values is tabs closebuttons restored. An add-on restoring the behavior of value=2 only is No Close Buttons.
Firefox 2.0 introduced a number of refinements to its tabbed browsing UI. Among them was the addition of close buttons on tabs. This preference controls how they can be displayed on tabs.
Possible values and their effects
Display a close button on the active tab only
Display close buttons on all tabs (Default)
Don’t display any close buttons
Display a single close button at the end of the tab strip (Firefox 1.x behavior)
First checked in
Has an effect in