For anyone else who comes here - short answer is that I was using HTTP
authentication, and UploadPlugin uses your password both for HTTP
authentication and to pass to the script. Making the password the same
worked - seems to be what TiddlySpot does.
Although I'm not sure about the wisdom of taking the same cleartext
password and putting it in even more places, which was why I'd had a
password in my store.php file that was different from the HTTP
authentication password.
As far as it working on Firefox, my guess is that it's a different
password caching implementation (perhaps Firefox tries the previously-
saved password even if supplied with a different one).
- Marc