Mozilla Firefox is disabling the browser’s backspace key to prevent users from accidentally losing data typed into forms. In 2014, Google removed the ability to go back to a previous page by using the key as it could cause the loss of data entered into forms on the current page. Firefox and Internet Explorer 11 are the only browsers to support this keyboard shortcut. To re-enable the key, you can change the about:config preference browser.backspace_action to 0. You can also use the recommended Alt + Left arrow instead.
Source: https://www.bleepingcomputer.com/news/software/mozilla-firefox-disabling-backspace-key-to-prevent-data-loss/