Sunday showings launch the Java interface, while most monday showings do not. It works for me. I can't verify whether the actual buying works though.

I have the official Sun J2RE and Firefox 0.8 (Mozilla.org build firefox-0.8-i686-linux-gtk2+xft.tar.gz).
Simon
EDIT: I checked again, it's not Java they're using but JavaScript (a completely different thing). Just enable JavaScript in your browser. I also tried changing the user agent string of my browser so that the site would think that I'm using MSIE. The seat selection stopped working, so there's some browser specific code used.