It's likely the Firefox developers use more of its shortcuts because they are more familiar with the product and are more interested in optimizing their workflow than a typical user. Exploring, which includes backtracking, is the core of the web browsing experience so I'm not surprised people are doing that far more than, say, searching for a substring.
It's also worth pointing out there isn't a great opportunity to learn the keyboard shortcuts Firefox provides. Traditionally you'd learn shortcuts by seeing them next to menu items, but the menus are hidden behind the Alt key, and many actions simply aren't listed there. For example, "Find Again" / F3 is omitted.
I'd assume that there is a class of users who never ever use any keyboard short deliberately (because they are not aware or dont care about their existence). Any keyboard shortcut they trigger is pretty much by accident. I have not data how common that is, but I know such users.
That seems to just be majority of users based on that stats listed above, and that backspace is the only one that is trivial to hit when trying to do something else.
It's also worth pointing out there isn't a great opportunity to learn the keyboard shortcuts Firefox provides. Traditionally you'd learn shortcuts by seeing them next to menu items, but the menus are hidden behind the Alt key, and many actions simply aren't listed there. For example, "Find Again" / F3 is omitted.