Don't even bother trying to use Snapfish with Firefox (or Mozilla, Netscape, Opera) if you are running on Windows. Just grit your teeth and load up Internet Explorer (IE) as annoying as it may be. Snapfish has a very nice uploading tool (an ActiveX control) that loads into IE and allows you to upload groups of photos from a directory easily. A lot of features, like ordering photobooks and the upload tool, just aren't available on Firefox.
I was trying to use Firefox and only go into IE when I had to for something like ordering a photobook. Unfortunately, you can't always tell what you are missing. If you are not in IE, you don't know you are missing the ActiveX uploading tool. If you are running outside of Windows I'm not sure I would recommend Snapfish at all. Shutterfly had more support for alternate browsers.
I made my feelings known to Snapfish with the advice that they support Firefox and other standards compliant browsers. This is a pretty big issue for me because IE is feels slow and ponderous next to Firefox (and firefox keeps getting better) but not yet big enough for me to drop Snapfish because I am happy with everything else.