I have been pondering why my Firefox installations all refused to login to a router at a DNS address, yet all my Internet Explorers worked fine. I was certain the login details were correct, especially as no invalid message appeared.

After looking through Wireshark to see what was going on I've eventually found it to be caused by the fact multiple cookies for the domain name in which the router sits on (reece-eu.net).

The session ID from the router is the last cookie in the HTTP cookie line because it was the latest created. However, the router login code only appears to take notice of the first cookie in the line. It sees that it is incorrect, so it then rightly redirects me back to the login page.

Having tried logging in with the IP address rather than DNS address I can successfully login with Firefox, due to there only being 1 cookie present from the IP address. I have also tried changing the password, that also works fine.

This is a bad workaround though, and the router software should really be amended to deal with this. It should take into account that there could be multiple cookies present for the domain name, and check each cookie in turn to see if one of them resembles a valid session ID rather than just see the first invalid cookie and not bother looking at the others.

So anyway... if you are accessing your router from a DNS address rather than IP address, this could be why you can't login even with the correct password.