Very simple add-on that closes the current tab and make Firefox forget about the visit: suppressing cookies related to the current page, cleaning (beware : may be partial) the browser history and so on.
Here is a little more about the add-on from the developers:
Close the current tab and forget about the visit : suppress the cookies related to the current page and, if configured, the whole domain from the history. After installing the extension, find the new “close” button in the toolbar customization panel and drop it in your prefered place. Optionaly, you can add an entry in the contextual menu of each page. Remember that you can also use <Alt> + W (or other) to close’n forget a tab.
Want to learn more? You can pick up Close’n forget from the Firefox Add-ons website.