embedded Firefox and basic authentication waits forever

No problem for me as IE works, but just to let you know - and maybe there's a trick you know:

I got a website that uses basic authentication. In that case, RDM will modify the URL to contain the credentials, i.e. http://user:password@site. That works with IE, but when Firefox is configured in embedded mode instead, the page will open but will remain empty.

When I switch to external Firefox mode, Firefox will pop up a confirmation box, saying that I'm logging in as the user XY on that site. So it's probably this confirmation popup which makes RDM wait forever in embedded mode. I've searched about:config with "basic" and "auth" and it seems that FF doesn't have a setting that would prevent this behavior. Maybe you know what it takes?

Best regards, Thomas.