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 Â Â Â Â (Default) Display close buttons on all tabs
2 Â Â Â Â Don’t display any close buttons
3 Â Â Â Â Display a single close button at the end of the tab bar (Firefox 1.x behavior)
1 Â Â Â Â (Default) Display close buttons on all tabs
2 Â Â Â Â Don’t display any close buttons
3 Â Â Â Â Display a single close button at the end of the tab bar (Firefox 1.x behavior)