My new SCC Basic shop was more or less finished today and ready for testing. For hours I was cursing because, preferring Firefox to IE, I was most upset when IE was handling the "add to cart" -> Proceed to Paypal.. procedure with no problems, but Firefox kept showing me either an empty cart or it allowed me through to the Paypal payment stage, but then Paypal told me that my cart was empty.
Cookies were switched on, so the problem wasn't there, but Firefox seems to treat cookies in a really peculiar way.
So what I did to solve the problem was,
- Tools
-- Options
--- Privacy Tools tab
---- To the right of "Accept Cookies from Sites". click the "Exceptions" button
----- Add the url of your shop (or anywhere else if you have the same problem elsewhere) into the field at the top and click "Accept".

Hope this information will help someone !

What is the URL to your shop Dane so we can look at it?

I would have to add to Scott and say we need the URL too as that shouldn't be that way, AND you certainly don't want it to be that way as most people will not know to do that and will leave your site without a purchase if it's not working correctly. I know if I can't get something to work on a site I leave and google another place to get what I'm looking for.

That fix is fine, works great, but it works for "you" not for your customers, they would have to do that too if it were a FF issue (which it could be since they just updated FF to 14.0.1 recently)

I had the same problem with Firefox, and it turned out to be as a result of the way i set up the url

From memory i think instead of the domain name being parked at my CPANEL it was forwarded

Might be worth telling support how your domain is set up, incase you have the same issue