So I’ve already pointed out features of Firefox 2 like tabbed browsing and detailed configuration. As you may have noticed Firefox 2 now features a Close button (that little X) for individual tabs. This is unlike in Firefox 1.X where a solo Close button is located at the right end of the tabs.

Here’s a cool tweak on modifying how the tab Close button behaves:

1. Bring up the in-depth configuration with about:config
2. In the “Filter” field type browser.tabs.closeButtons
3. Double-click on the entry and enter a number below:
   0 - Display tab Close button only on the active tab
   1 - Display tab Close button on each tab (default)
   2 - No tab Close buttons will be displayed
   3 - Display a single close button at the end of the tab bar (a la Firefox 1.X)
4. Click OK.
5. Close the configuration tab or window.

Source: Firefox 2.0 Close Tab Buttons