from:http://www.bleepingcomputer.com/forums/topic389967.html
Or, if like me, you don't like having that little "x" taking up space on the tabs, you can move the Close Tab Button to the end of the Tab Bar, or you can do away with it all together, if you prefer to center-click tabs, to close them.
Double click the blank space, in the Tab Bar, or right click a tab and select New Tab.
This will open a new, blank Tab.
In the Address Bar, type; about:config
Click Enter.
Scroll down to, and double click, browser.tabs.closeButtons.
This will open the Enter integer value dialog box.
Change the value to one of the 4 Integer Values listed below, depending on your preference.
Click the OK button.
Exit about:config.
Restart Firefox.
Integer Values:
0 - Display a close button on the active tab only
1 - Display a close button on each tab (default)
2 - Don't display any close buttons (if you prefer clicking with the center mouse wheel, to close tabs)
3 - Display a single close button at the end of the tab bar (Firefox 1.x behavior)