Having done a few designs where I ended up pounding my head against the wall trying to figure out what was causing page shift–where the content seems to jump to the left or right on certain pages–I understand this can drive a poor designer insane.

It’s caused by inconsistencies between how browsers handle the vertical scroll bar combined with centered content. Some browsers, like Internet Explorer, display scrollbars on all pages, while others, like Firefox, do not.

Adding this code to your stylesheet will correct the issue.¹

html { min-height: 100%; margin-bottom: 1px; overflow: -moz-scrollbars-vertical !important;} 

1. The “-moz-scrollbars” declaration takes care of this issue in Firefox 3.5+