You can customize the display and location of the close tab button in the tab bar by using about:config to edit the preference browser.tabs.closeButtons.
Values:
0 Display a close button on the active tab only
1 (Default) Display close buttons on all tabs
2 Don’t display any close buttons
3 Display a single close button at the end of the tab bar (Firefox 1.x behavior)
1 (Default) Display close buttons on all tabs
2 Don’t display any close buttons
3 Display a single close button at the end of the tab bar (Firefox 1.x behavior)