The default for Firefox on Linux is to map the backspace key to go one viewport up the page (Shift+Backspace will bring you down). On other OS it will go back to the last page you visited within that tab.
Since I have a Page-Up key conveniently placed beside my cursor keys, I like to have it go back to the last page instead. Luckily you can. The config is a bit hidden, but it is there. Go to
about:config
There change the value of browser.backspace_action to 0 to enable backspace to go to the last page. Value 1 will enable the viewport
up and down functionality.