I asked my partner, who is more knowledgeable about hardware than I am, to take a look. What follows is his research.
In looking at the page source using the "view source" option, I see that the headers are very sparse or mostly nonexistent. Most particularly, the first line that is present in the source code for this forum page, namely
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd
is missing. This causes a Known Problem in some browsers; it puts them into "quirks mode", with variable and sometimes unpredictable results. In this specific instance, it appears that the missing line causes the center background to fail to extend to the bottom of the page in both Firefox and Seamonkey, on three different systems here. The exact break point for the transition from grey to black varies, but it happens with all three machines in Firefox, and in Seamonkey on the machine which has that installed - and Seamonkey is based on the same core code as Firefox.
The solution is almost certainly to add that missing doctype declaration at the top of the page. Yes, I know, it's structurally Just A Freaking Comment, but apparently Firefox READS that comment and makes decisions based on what it sees.
I should note that this was the first time I'd looked at an episode page since upgrading to Firefox v13.0.1, so the problem may be extant in other episode pages as well. He says that this is the longest single episode page you have, and apparently the only one long enough to trigger the problem.