If you're getting stuck in an endless loop when trying to login to your Wordpress control panel using Firefox, and you're sure you're entering the right credentials, the issue is most likely a bug in the coding related to cookies.  I've lost hours and probably a lot of hair trying to overcome this, but thanks to a post by richtelford buried deep in the Wordpress forums (http://mu.wordpress.org/forums/topic/11865?replies=2) there is a fix.  A painfully simple one…

FTP to your server / Wordpress installation folder and open the file wp-config.php in the root.

At the very top of the file (after the opening PHP declarations) paste the following code:

@define('ADMIN_COOKIE_PATH', '/');

Save the file, re-upload if necessary and the problem should now be fixed.