SSL weirdness

So I’ve made this online store. http://tinyurl.com/36g5cd.

Got an SSL certificate for it and all, but during checkout Firefox reports it as being only ‘partially encrypted’ (crossed-out padlock symbol).

I think it’s because I’ve got a whole bunch of Session variables hanging around (for things like Currency selection, last page viewed, etc) but the actual sensitive details are all POST’ed to the secure area.

What am I doing wrong?