Back IS Close replaces the “Back” button with a “Close” button when the “Back” button is disabled because there is no page to go “back” to. The close button will close the tab or browser, as appropriate.
Here is a little more information about this unique add-on for Firefox:
Back IS Close is the perfect complement to tabbed browsing: it closes the many tabs tabbed browsing opens. Back IS Close works most effectively when you map a mouse button (I use the right mouse button) to send “Alt-Left Arrow”. Then one mouse button will either take you back to the previous page, or close the tab if no previous page exists, making for quick, convenient browsing.
Want to learn more about Back IS Close? You can pick up this add-on for Firefox from the Firefox Add-ons website.