ANSWER:

I have the same router and the same issue, I think the reason might be detailed in this post: http://support.mozilla.org/en-US/questions/918127

Response by cww:

There was a security protocol change that was introduced in Firefox 10 (as well as in Chrome 18 and in IE9 with the latest security patch from Microsoft) to address a particular vulnerability. Some poorly coded sites/pages are taking a shortcut when interpreting the secured data stream and now that the format has changed they don’t handle the change gracefully and bail on the connection.

As for options, you can see if the router will accept non-HTTPs connections which should be fine or you can try to contact Cisco to make a firmware update to support this new protocol.

by Thien from http://superuser.com/questions/414529