This tutorial explains how to set the default action assigned to the Backspace key in Firefox. You will notice that pressing Backspace (when not in a text field) brings you back to the previous webpage you visited, which can be quite annoying!
Type about:config in the address bar of Firefox and press Enter. Search for the browser.backspace_action preference:
The default value is 0, which means that the backspace key will return you to the previous page. We can edit this value by right-clicking on the entry > Modify:
Two other choices are available:
Once you have changed the default value, click on OK and exit the Preferences window. The new settings will be seamlessly applied, there's no need to restart your web browser.