The standard Google Apps URL for your domain’s Control Panel (https://www.google.com/a/DOMAIN.NAME/) has a nasty redirect-loop, preventing successful login:
Firefox has detected that the server is redirecting the request for this address in a way that will never complete.
Technically, you are receiving a HTTP status 310, redirect error:
Error 310 (net::ERR_TOO_MANY_REDIRECTS): There were too many redirects.
Fortunately, there is another way to access your Control Panel: log on to Google Mail using an Administrator-account for your domain:
There, you should find a “Manage this domain” link in the upper right hand corner. It will take you to the Control Panel.