credential files

[m.roth2006 at rcn.com]

Does anyone have any idea how a browser recognizes that a cite is asking for a credential file, and hands it back to it?

For example, I go to a site, and firefox suddenly says "this sites wants a credential - is this the credential that you want to give it?" I've used a plugin that shows me http headers and responses, and see nothing where that happens.

Links? Pointers? Clues for the poor?

        mark