That's interesting as I have not changed that link, it's as it was when I first created it, so I would have to say that the new Firefox v4.0.1 isn't loading that page properly as I think you've already assumed.
Now somewhat in defense of Firefox v4.0.1, I have to point out that the page in question is NOT a 'static' web page as the Threaded Index page is, but instead is a 'dynamic' web page that has to be sort of 'configured' on the fly as it loads each time.
What I mean by that is that it's a .cgi (Perl) script that runs on my web host's server to first count all of the message files on the server, then allocate the latest (newest) 100 message files to links in the main page that your link is for, then the next 100 message files for the 2nd page that isn't yet loaded but is used to determine the [Next[ navigation link in each of the Date Index pages that can be loaded.
This process is repeated throughout all of the available message files until the last (oldest) message file is processed for the [Oldest Posts] navigation link in each of the Date Index pages that can be loaded. Then, and only then can the main Date Index page be loaded.
This process may seem to require a lot of time, but in reality it's finished in a matter of seconds, or less, depending on external conditions of the Internet at that time.
That being explained, I will state that this is a common task for web servers as it's been since these scripts were first used, so I have to associate the issue with the new Firefox v4.0.1.