Jeeez! The more I think about this, the more it's beginning to look like a compatibility issue of the Amazon site with various different browsers - but apparently not with Chrome????. I wonder how thoroughly those who tried using Chrome have actually tested. Is Chrome truly an outsider in this regard?
Perhaps, and especially in view of us not being able to pin this down any further, we should all forget about running any more experiments (deleting this and that and plists in Safari) and just wait and see whether in time the issue goes away. Annoying, I know, but if it's due to a subtle bug in the way that the Amazon site now works, we could have already been wasting a great deal of time and upheaval for no good reason. I myself resent that I've had to delete and reconstruct so much on my Mac, but all to no avail.
Perhaps all we need to do is be patient and the techies at Amazon will eventually discover something and then put it right? I have to concede, though, that it's going to make the using of the site quite tricky in the interim because of the somewhat random occurrence of the beachballing. Last night, for example, I went to the site and deliberately browsed and clicked in various depts for maybe 10 minutes. No beachball. I then came out of Amazon, closed Safari completely, then re-opened Safari and went back to the Amazon homepage. On the first click there I instantly got the beachball.
Just because it's at times looked like Safari has been at fault doesn't, in the absence of any further evidence, mean that it is necessarily due to Safari itself. But if it's true that the 'crashing' also happens in Firefox (and presumably that's Firefox for Mac) and we know that it happens uniquely at the Amazon site, then the finger begins to point toward the site itself, doesn't it?