Pressing Backspace while placing tiles in Firefox on Windows goes back to the previous page (like hitting the Back button). The following steps will disable this feature in Firefox:-

• In the address bar, type "about:config"
• Accept the warning
• Search for "browser.backspace_action"
• Change the value from 0 (which means go back) to 2 (don't do anything)

References:

• http://kb.mozillazine.org/Browser.backspace_action
• https://support.mozilla.org/en-US/questions/1194739