When navigating from a page that fits your browser to one that overflows you can see page shifting. Here's a tip from Lee White to make the vertical scroll bar always visible to prevent the shifting.
Using Windows Explorer go into the \chrome folder using
%APPDATA%\Mozilla\Firefox\Profiles\
Select the oddly named AAAAAAAA.Default folder. Now select the Chrome folder. If you don't have a userContent.css file you should have userContent-example.css so make a copy of it and rename it to userContent.css.
Open it with an ASCII editor and add the two lines below...
/* Visible vertical scrollbar */
:root { overflow-y: scroll !important; }
Now when you restart Firefox your vertical scrollbar should always be visible as it is in IE.