From Birnam Designs Wiki

To modify the display of the X close button on each tab, go to about:config and edit browser.tabs.closeButtons to one of these values:

0 - Display a close button on the active tab only

1 - Display a close button on each tab

2 - Don’t display any close buttons

3 - Display a single close button at the end of the tab bar (Firefox 1.5 behavior)