Restore BACKSPACE functionality to take you to the previous page in Firefox on Linux

1. Type “about:config” in the address bar of Firefox and press Enter.
2. Type backspace in 'Filter' and look for ‘browser.backspace_action’
3. Change its value to 0 (zero).

More information

Background

The backspace key was mapped to the browser ‘Back’ function in Mozilla for consistency with Internet Explorer. However, to improve consistency with other applications running on Linux, it was decided that this mapping should be optional—and set based on which platform the browser was running on. As a compromise, this preference was created to allow the backspace key to either go back/forward, scroll up/down a page, or do nothing.

Possible values and their effects

0:
Pressing [Backspace] will go back a page in the session history and [Shift]+[Backspace] will go forward. (Default in Windows)

1:
Pressing [Backspace] will scroll up a page in the current document and [Shift]+[Backspace] will scroll down. (Default in Linux builds before 2006-12-07)

Other:
Any other integer value will simply unmap the backspace key. In Linux builds after 2006-12-07, the default is 2.

Reference: http://kb.mozillazine.org/Browser.backspace_acti