I am using Firefox 4.0.1 as my primary browser, and am testing Internet Explorer 6.0.2900 with SP2 as we speak. Neither has worked today. I have not tried another computer yet.
Not a big deal, and I still suspect some software on my end trying to save me from something evil. I have had something odd before that turned out to be my Computer Associates security suite's firewall software. I tried after disabling that and it still made no difference.