Firefox for Windows users can disable the back function mapping to backspace key, and Firefox for Linux can enable the backspace to map to Back function easily.
Here's how to do it:
1) Type about:config in Firefox Location Box, and then confirm the warning message.
2) Locate the following preference name:
browser.backspace_action
3) Change the “browser.backspace_action” to one of the following values:
0: Pressing [Backspace] will go back a page in the session history and [Shift]+[Backspace] will go forward.
That's your default in Windows.
1: Pressing [Backspace] will scroll up a page in the current document and [Shift]+[Backspace] will scroll down. Except Camino that does not implement any behavior for the value 1, which is unmapped. (Default in Linux builds before 2006-12-07)
Any other integer number: Any other integer value will simply unmap the backspace key. In Linux builds after 2006-12-07, the default is 2.
Hope this helps!