I would just like to point out that your point and the parent's point are both valid and non exclusive.
As a example, one can navigate back and forth by gesture or shortcut keys, and never have to use the back and forth buttons. Still, the display of the buttons to conveys the presence or not of back and forth history can be useful.
Fortunately, this is Firefox we're talking about. I'm sure there will be an about:config option to disable the new behavior if you don't like it. Just like you can change browser.urlbar.trimURLs in Firefox 7+ to restore "http" at the beginning of URLs.
Even less users use the huge orange FIREFOX button that uses up vertical real estate, but they didn't hide/remove/move that? I see no reason it should be where it is, as big as it is and as bright as it is. Put it where chrome puts it.
40% of users never touched the forward button even once.
Another way to interpret that data would be "60% of users at least sometimes use the forward button."
The back-forward pair of arrows conveys a very intuitive idea for most people. In the absence of a "forward" button, I wonder how many FF10 users will accidentally click "refresh" thinking that it's the "forward" button. After all, the arrow in the "refresh" button points in the opposite direction of the arrow in the "back" button, and now the two buttons are side by side. It's not difficult to imagine them as a pair. Yeah, one arrow is straight and the other one is curly, but I often think of "forward" as "undo back", and many programs represent undo/redo with curly arrows.
The refresh button has moved since that diagram. It, along with the stop button, now lives on the right edge of the location bar, far away from the back button.
Here's one study, not sure if it is the most recent: https://heatmap.mozillalabs.com/
40% of users never touched the forward button even once. And those who did use it, didn't use it very often.
Several of the very low% UI elements (like RSS or Go) were removed completely from the default setup.