Weird rendering issues in Firefox on my @squarespace site

I noticed some off rendering issues in Facebook when I view this site in Firefox (pretty much the last few versions), compared to Chrome. This is what this site looks like in Chrome:

This is what the site looks like in Firefox (I’m running the stable version 11):

I notice 2 issues in Firefox’s rendering. The first is that the grey background visible in Chrome gives way to a black background in Firefox. The second is the horizontal scrollbar at the bottom of Firefox suggesting there is more on the page to the right of the visible area (there isn’t).

I contacted Squarespace Support about this a while ago when I was running a beta version of Firefox and received the usual response that they don’t support beta browser versions but this issue has persisted. I’m going to point them to this post when I submit a ticket but I’m curious if anyone else has any ideas about possible causes.