What function of the backspace button is this about and what is failing ?

This is the meaning of the pref:

• about:config => browser.backspace_action :: 0:Back/Forward; 1:PageUp/PageDown; 2:Nothing

Thanks but that's exactly what's not working for me

This B/F action would only work when no other element that can take key presses has focus. Do any of the other values work?

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Navigating back using the Backspace key works for me with browser.backspace_action set to 0 in about:config.

Does it still happen in Troubleshoot Mode?

Thanks all. Neither of the other values work either. The issue still exists when in Troubleshoot mode. Weird.

Is the toolbar Back button highlighted and is there tab history if you right-click either B/F buttons of hold the left mouse button ?