From MozillaZine Knowledge Base
In conjunction with several other improvements to tabbed browsing in Firefox 2.0, close buttons were added to tabs. These appear on the right or left side of the tab, depending on your OS. Since the close button takes up 16 pixels’ worth of tab width, it can obscure the tab’s text if you have a lot of tabs open and the tab widths become narrow. As a compromise, the close buttons are only shown on background tabs if their width is equal to or greater than this preference’s value.
Possible values and their effects
Any positive integer value of pixels. When a background tab has a width equal to or greater than this value, a close button is displayed on the tab. A value of 0 means that each tab will have a Close button. The default value is 140.
Values less than 50 may prevent any text from being displayed on especially narrow tabs. Large values (greater than the width of your browser window, for instance) effectively disable the tab close buttons on background tabs, excepting the behavior described in bug 324259.
First checked in
Has an effect in