Edit: developer response puzzled me, so I searched and found that indeed the functionality this extension provides is still part of Firefox; it just needs to be enabled in about:config. Change "browser.backspace_action" value from 2 to 0 (zero). And it works! Without the icon in the URL bar or need for this extension. So I'm still puzzled by the developer's response.
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Works well. Would it be possible to remove the indicator icon in the URL bar, or to make it optional? The icon appears regardless of whether there is history to go back to or not, so it is not really serving a purpose, and really the back arrow already serves the purpose of showing whether or not there is history to go back to, so the only purpose of the icon is to show whether or not the extension is enabled. Most people are going to keep it enabled all the time, so this is basically just visual clutter. Please consider this; thank you!

Unfortunately this extension works only on half of pages visited. I can't tell you in which pages it works or it doesn't but surely it does not work in many pages, and that's why I give it only 2 stars. The creator should recheck it and correct it because it's a great help for experienced users that work more with keyboard and less with mouse.