Applet: Login problem with Firefox (but not with MSIE)

Whenever I try to upload pictures with Firefox using the applet, GR just opens a dialog asking:
"Enter username and password for realm RP6<xxx> on host www.digitalbild.de:80. Authentication Scheme: Basic"

And no matter what (valid) User/PW is entered, no login is possible.
With MSIE everything works fine.

btw.: The problem ist exactly the same with Gallery2.

Searching the forums and FAQs didn't offer any answers, so I hope my question isn't too stupid.

Thanks a lot,

Robert

Gallery URL: http://www.digitalbild.de/gallery
Gallery version: 1.5
Apache version: Apache/1.3.29 (Unix)
PHP version: 4.3.10
Graphics Toolkit: NetPBM
Operating system: ?Linux? (Don't know, what my hoster "1&1" is running)
Web browser/version: Firefox 1.0.4