Look for a file called "browser.xml" or "browsers.xml". Some guy told me that you can change there the browser (for ASP.NET in Whidbey). I have installed ... It's somewhere under "Documents And Settings\<User>\Local somewhat\AppData\VisualStudio\8.0"
To ged rid of the messagebox asking you the password and user you have to deactivate in Firefox under
Websites->Start Options->Start Options
the point saying "NTLM Authentication".
Cheers
Christian