You need to change your settings in about:config. Here’s what to do: Enter about:config into the address bar (if it gives a warning about your warranty, click the button to say you’ll be careful) Enter layout.scrollbar.side into the search box at the top of the page Double-click the result and replace the current value with a 3 when the prompt comes up Click ‚OK‘ and then restart Firefox.

Quelle: HOW DO I MOVE THE SCROLL BAR TO THE LEFT SIDE? | Firefox Support Forum | Mozilla Support