In Firefox, if the webpage you are viewing is not taller than your screen (and doesn’t need a scrollbar), Firefox will completely remove the scroll bar from your screen. This adds 20px of more space to your view space. Unfortunately, if your website is centered, this will actually move your webpage 10px to the left or right when switching between some pages.
This CSS code will automatically force a sidebar onto every page of your website; even if the scrollbar is not needed. That will stop your webpage from moving left and right on different pages:
html { overflow-y: scroll;}
Enjoy,
Ashton Sanders