Back IS Close
Back IS Close is a Firefox Extension.
You can download it from the Firefox Add-Ons site.
Normally, your browser’s “Back” button is greyed out when there are no more pages to go back to:
This extension replaces the greyed out “Back” button with a “Close Tab” button:
Clicking on this button will close the tab that you’re currently using.
You can even use this button to close the entire browser window, but you’ll have to set it to do so in the options settings:
