Authentication Servers Down When Others Can Join Just Fine?

When I join a multiplayer server, it says that the Authentication Servers are Down, even though others can join those multiplayer servers just fine. I checked the Statuses on minecraft.net and they're fine as well. What could be the issue, then?
The only issue I can think of if it is related to a firewall blocking this access. This may explain why this incident has been going on for much longer than it should. Other than that, any ideas...?
Thanks!

-Kyle