Browser Close Button Options in Firefox 2
Close buttons on each tab (default)
browser.tabs.closeButtons = 1
Close button at the end of the row of tabs (Firefox 1 style)
browser.tabs.closeButtons = 3
Close button only on the active tab
browser.tabs.closeButtons = 0
No tab close buttons (use Ctrl+w or right-click tab to close)
browser.tabs.closeButtons = 2
For more information see the mozillaZine Knowledgebase article
Browser.tabs.closeButtons.