This extension brings a great feature to Firefox that's been in mobile Safari for some time now: the ability to use the "Back" button (or shortcut key) to close the current tab and return to the tab it was originally opened from. This works for "newly opened tabs" like the description says, and also when at the beginning of a tab's history. Like browser tabs themselves, once you've used this feature, it's hard to go back to a time without it.