Mozilla Firefox: Close button on tabs
  1. Enter about:config in the address bar.

  2. Search for browser.tabs.closeButtons

  3. Possible values are:

    • 0: Only the currently active tab displays a close button.
    • 1: Each tab has its own close button (Default).
    • 2: Tabs without close button, a tab can only be closed via context menu or Ctrl+W.
    • 3: Only one button (on the right) for all tabs; acts on the currently active tab.

Source: c’t 2009, #23, S. 166.