Fixed the link, I'm a moron.
Both I and Aves2k have verified that it's doing this. In both IE and Mozilla, so it's not a mozilla specific issue. It looks like the browsers are loading a new copy of every single image into memory every single time that it shows up on the page, even if it's already loaded and being displayed elsewhere on the same page. I don't know if that's by design (which would be really stupid) or due to the code used to display the page.
Even if you disable images in either browser, they both reach over 300MB memory usage. The browsers are obviously still loading images even when you tell it not to display them (way to save bandwidth and speed page loading by disabling images).
To make it do this, you DO have to be running in the enhanced graphics mode, and have your preferences set to display all posts on a single page.
What's even better is how well that thread shows IE's horrible underlying coding, even though overall it's fast and looks good.

When I let the page load in IE6, then close the window, it takes a good minute to two minutes for IE to finish unloading from memory and doing whatever cache and history stuff it needs to do, during which is uses 100% of the CPU time and causes other activities on the system to lag. When I close Mozilla with the thread open, it closes instantly and the process is gone.