This is needed for UK keyboards because the right "Alt" key is "AltGr".
Pressing AltGr+LeftArrow does not go back to the previous page as I prefer.
I have to either use both hands, pressing the left "Alt" and LeftArrow to go back, or re-
enable the Backspace key in about:config, by setting "browser.backspace_action" to 0
which I have done, to allow me to use the Backspace key as before.