This is what I get on FF 11 on ubuntu.
I cannot recreate this using Firefox 12 on Mac.
Are you sure this isn't related to an extension? Subsection Tabs for example modify the padding/margin of #contents (but it shouldn't of course modify the pages area).
I have no extensions installed. Same happens on Chromium 18.
In 2.3, it displays correctly for me in both Firefox 11 and Chromium 18, Ubuntu.
Sorry for the fake bug, I made a quick test with a fresh install and it works fine (not sure what happened though).