By default, Firefox displays a close button on every tab. But you can hack this and change according to your requirement.
So let start now with these  steps (with Firefox 3.03)

Type in your location bar – about:config and Enter
you will see a message – Click on I’ll be careful, I promise (if your open first time)
Then in the filter bar type – browser.tabs.closeButtons
Double click on browser.tabs.closeButtons
By default value is -1
Firefox display a close button on every tab also depend how much tab you have open.

When you open firefox on full screen-

Maximum open tab with close button – 5 (800 x 600 screen resolution)
Maximum open tab with close button – 7 (1024 x 768 screen resolution)
Maximum open tab with close button – 9 (1440 x 900 screen resolution)

If you open more tab according to your screen resolution, Firefox display close button setting automatically changes and now Firefox displays a close button only on the currently open tab.

Set default value – 0
Firefox display a close button only on the currently open tab.

Set default value – 2
Firefox don’t display close button on any tab.

Set default value – 3
Firefox don’t display close button on any tab but it show end of the tab bar in right side.

Note: Don’t change about:config setting without full confidence & knowledge

Tags: Firefox, firefox-hacks