Javascript works just fine in Safari... in fact, I've only come across one site that did broken and wrong things that didn't work in Safari, compared to dozens that don't work in Firefox (which I used for a year and the only good thing I can say about it is, at least it's not IE).
Not sure why the different experiences... I do know it's nothing to do with Safari being "exclusive", but sites using broken javascript that some browsers may "enable" by trying to guess at what it's supposed to do. Or just letting it run as it may even if it's incorrect, which is a huge security and usability hole.
I was trying to test it to let you (and the site designers) know what the actual problem is, but it's taking god knows how long for their registration confirmation email to get through, so.... first impression, not so hot.