It works most of the times, but sometimes it only goes back one page. For example, if you are using Google Maps, this button only goes back one page, not to your startup page. I still have to click on the back arrow pull-down to go back all the way. At other times, it does nothing (for example, the page you are reading now, https://addons.mozilla.org/en-US/firefox/addon/snap-back-button/reviews/ -- doesn't go back at all).