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:
1: Use the Backspace key to scroll to the top of a webpage (Page Up)
2: Disable any special action assigned to the Backspace key
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.