Problem Connecting to Certain Websites

[Judge Dee]

I haven't been able to reach 2 websites on my computer ("Down for Everyone or Just Me" and "Rifftrax"). I can reach them on my wife's computer who's on the same network.
I read about changing the MTU settings (whatever they are) which I did, but still no luck.
Nothing in Hosts except Wilders.
No blocking software of any kind except AdBlockPlus, which I disabled.
Any suggestions?

Ubuntu 10.04, Firefox