If amazon knows ( a cookie is set on the browser that indicates ) that you are a returning user, the login when entering checkout lets you enter your email address and password just fine. However if you jump over to a different browser ( or computer ) that you haven't used and try to preform that same action you will receive this error:

You clicked on the button indicating you're a new customer, but you also provided a password. If you're a new customer, please do not enter a password yet (we'll ask you to create one later). If you're a returning customer, please click on the button indicating that you already have a password, then type in your password.

I emphasized the first phrase. I DID NOT click on any such button. The rest of the error message is helpful, but I dislike being jolted from my normal routine by an accusation that I've done something that I haven't done.

My main frustration though is the reason that I had to log in in the first place. I'm doing something for work with an amazon seller account and doing something else with my personal account. There is no way to do this in separate tags or windows of the same browser. I find this frustrating.

one solution would be for companies to allow multiple people to be logged in simultaneously, which would obviously cause much confusion and clutter the interface with identity select boxes and such. A better solution in my opinion would be a "create tab with new cookiespace" option so that I could browse as an unidentified user... I could then possibly have the option of saving the cookies generated in that session as a cookie profile or some such.

maybe such an addon already exists for firefox... maybe I could write one. hmmm.