This is not a Mozilla bug. This is a problem with the admittedly clueless webmaster of that site. If it's absolutely vital that you use their broken website, change the user agent that Mozilla sends. Info on how to do that is here: http://www.mozilla.org/unix/customizing.html
I filed this bug in the "Tech Evangelism" section, which is, as far as I am aware, intended for use to point out clueless webmasters needing to be given a clue. Why then is it considered "invalid"?
Dan you are absolutely correct. -> REOPEN Christopher, please do not mark Tech Evangelism bugs invalid. If you have any questions regarding Tech Evangelism contact me or come to #evangelism on irc.
Oops, I neglected to look in what section it was filed. Completely my fault. My apologies.
tech evang june 2003 reorg
The site in question has been redesigned, and no longer redirects Mozilla users to the rude "upgrade your browser" page.