Close The Current Tab on Firefox By Double Click
Close Tab by Double Click is a simple lightweight Firefox addon that does just one thing: lets you close a current tab with two clicks of your mouse.
To prevent the last tab from closing, you could modify the following property in about:config: “browser.tabs.closeWindowWithLastTab” that is true by default.
A note for the “LastTab” users. You probably could note that annoying tab flipping when double clicking a tab. To fix that you could uncheck item “Focus last selected tab when current tab is clicked” in LastTab’s options.
Works with Firefox 1.5 – 4.0b8pre