I have noticed that several times when I visit the index pages using firefox, the page remains white rather than forwarding the browser to /perl-bin/animedb.pl. However, once I plug in the url directly, it works.
You must probably have an extension filtering such redirections or anything related. Disable all them or check with an extension free installation. I don't have that problem with AniDB and I use a lot of extensions but had similar problems before with Adblock.