In the past I've come across two great links on tweaking Firefox configuration settings, that I had to refer to yesterday again.
The first one is about how to do automatic Windows Authentication with Firefox. If you have been trying out some local website that requires Windows Authentication in IE & Firefox, you may have noticed a login box asking for your credentials appearing only in Firefox. To turn that off, type about:config in the address bar, locate the Preference Name network.automatic-ntlm-auth.trusted-uris using the Filter option.
As mistyping anything in the configuration settings accidentally can harm the stability of your Firefox browser, you get the following funny warning in Firefox (click on the image below to see larger one) -
Double click on it & in the dialog box that opens enter it's string value as "localhost" or whatever is the website address that's pestering you.
It's not easy to understand all the about:config entries, so there are a couple of friendly visual editors to help you tweak the settings.
Also see: