Three simple but powerful features:
1. Click middle mouse button on an empty space on the page to go back. (Middle-Click on a link still opens it on a new tab.)
2. Click middle mouse button on an empty space on tab bar to repoen the last closed tab. (clicking middle mouse button on a specific tab still closes that tab.)
3. Click middle mouse button on reload button to duplicate the page. (i.e. to reopen the page on a new tab.)
https://addons.mozilla.org/en-US/firefox/addon/5938
