This code make possible to view every close button firefox can show.
There is an option in about:config
browser.tabs.closeButtons
0 - Display close button only on the active tab
1 - Display close buttons on every tabs
2 - Never display close buttons
3 - Display single close button at the end of the tab strip
So my code is like to have 1+3 preference stablished.
UPDATED 4 April, 2008 => Now it works for Firefox 3 too.