Firefox Extension Guru's Tweaks

Close Tab Buttons
Changes the way the Close Tab buttons are displayed

Tweak Type:	Display \|Tabs
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

One of the new & improved features starting with Firefox 2 is having the ‘Close Tab’ button on each tab. However, there are folks who don’t like change and would rather have the close tab button at the end of the tab bar as it is in Firefox 1.x builds. Sure there are extensions that will do this for you, but why bother downloading an extension when all it does is make a change to a setting in about:config? A simple about:config tweak will change the way the ‘close tab’ button is displayed:

Click the about:config button (if about:config extension is installed) or open a new tab and type about:config in the address bar and press enter
In the filter field type browser.tabs.closeButtons
Double-click on the entry and enter the corresponding number below:
Click OK
Close the about:config editor or tab

Note: If you are using Tab Mix Plus extension, this tweak will not work.

Source