If you want to make the Backspace on your Firefox browser working and gets you to the previous page you will need to open the browser and then type in the address bar “about:config”
Then you can type in the Filter field: browser.backspace_action
Once you have it you need to change the value for it to 0. It will look like this:
If you have done it correctly when you press the backspace button on your keyboard you should be able to go to the previous page.