In Firefox, backspace has defaulted to "scroll up" for years now and yes, browser.backspace_action in about:config controls this.

My Firefox has browser.backspace_action set to the default value (zero), and it goes back to the previous page when I press the backspace key.

It's been less than 36 hours since I lost several hundred words of work because of this, so I'm pretty sure the default behavior is "go back" as of the latest stable version (46.0.1).

Maybe they switched it back at some time. I remember changing it from 1 or 2 to 0 on few installations.

It seems like 0 (go back) has always been the default on Windows builds. On Linux it used to be 1 (scroll up) but they changed it to 2 (do nothing) in 2006.

http://kb.mozillazine.org/Browser.backspace_action

I made a half-assed attempt to migrate my Windows Firefox profile to Linux a while ago, which probably explains why I'm seeing the Windows behavior on both platforms :(

There was a Firefox patch posted just last week to make Linux's backspace key behave like Windows and OS X:

https://bugzilla.mozilla.org/show_bug.cgi?id=986982#c6