A friend pointed this out to me:
http://kb.mozillazine.org/Browser.backspace_action

In short, if you’re a GNU/Linux user of Firefox (e.g. Firefox on Ubuntu and Iceweasel on Debian) you may want to get backspace to actually go back one page, like many browsers do (did?):

1. In addressbar, type about:config
2. If asked, confirm you want to change settings
3. In search, type browser.backspace_action
4. Set the value of browser.backspace_action to 0 (that is zero, not letter O) Zero is Windows default and makes pressing backspace go back in history; One is old Linux default and scrolls page up; Two is new Linux default and, like any other integer, simply unmaps the backspace key.

Now, enjoy pressing backspace to go back!

Image source

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via blog.vucica.net