The bottom part flickers and overlays the content
This is a Firefox performance issue; I am aware of it. The flickering is a very minor problem with a reasonably powerful machine but can be ugly with slow ones.
Try
Opera 10.6. It is fully standards compliant, as Firefox, but much faster. There is absolutely no flicker with Opera navigating our website. I must say the same about Konqueror, but unfortunately it is part of KDE and hence is only available on X11.
Interestingly IE shows the page much better except it thinks there's no end to the page so you can keep scrolling downward
This is an IE bug. The problem is that IE does not know how to compute the true page height when the body element occupies the whole available space, and this property has been set through CSS. The height CSS property of both the html and body elements is set to 100% in pixinsight.com. I know this problem and will try to find a solution when I have other priority tasks completed (as 1.6.1 for example). I think this isn't urgent because other than being a bit surprising it doesn't prevent the user from navigating the site. Safari has exactly the same bug, by the way (surprising?).
Seems someone got a little fancy with the css
Indeed. If you analyze pixinsight.com internally you'll see it's quite sophisticated. I still have to tune it to work well with a few misarable browsers, that's all.