Browser Close Button Options in Firefox 2

Close buttons on each tab (default)
browser.tabs.closeButtons = 1

Screen shot showing close buttons on each tab

Close button at the end of the row of tabs (Firefox 1 style)
browser.tabs.closeButtons = 3

Screen shot showing close buttons on each tab

Close button only on the active tab
browser.tabs.closeButtons = 0

Screen shot showing close button only on the active tab

No tab close buttons (use Ctrl+w or right-click tab to close)
browser.tabs.closeButtons = 2

Screen shot showing no tab close buttons anywhere

For more information see the mozillaZine Knowledgebase article Browser.tabs.closeButtons.