Why does my content shift to the left on some pages (in FF)?

That'll be the pages with more content? The ones that have a vertical scrollbar? If you look in IE there's probably a white space on the right where there would be a scrollbar if there were enough content to require one. In Firefox, the scrollbar appears when it's needed and the viewport becomes about 20px smaller, so the content seems to shift to the left when you move from a page with little content to one with lots of content. It's not a bug or something that needs to be fixed, but it does confuse and irritate some developers.
If, for some reason, you'd like Firefox to always have scrollbars - whether they're needed or not - you can do this :
CSS html {