[throwaway8941]

I didn't even know this "feature" was available on Firefox (despite using it since version 2). Turns out, it is only enabled on Windows:

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

Maybe it was copied from IE to keep "closer to the platform"?

[rootusrootus]

It's definitely not limited to Windows. I am exclusively a MacOS user (for my workstation, at least) and I have had to disable it on any new Firefox install.