After upgrading my wife to Firefox 2.0, she complained to me that the Costco quick upload feature was no longer working. She had to upload her photos the painfully slow way.
It turns out if you change your User Agent to Firefox 1.5, Quick Uploader will work. Here is how to get Snapfish/Costco's Quickloader plugin to work with Firefox 2.0.
1) Type about:config into the url bar.
2) Right click on some whitespace and select New->String
3) Enter general.useragent.override for the preference name.
4) Enter you user agent string for the value. I used Mozilla/5.0 (Windows; U; Windows NT 6.0; pt-BR; rv:18.104.22.168) Gecko/20060909 Firefox/22.214.171.124
5) Restart Firefox
6) Type about: in the url bar. You should see your new user agent string in the build identifier.
Be warned: this changes your user agent for all sites.
UPDATE: Costco seems to have fixed this problem as of 2006.12.05