It seems to be real random. I've had intermittent 404's, fewer today as time goes on.
I was just now talking with my wife on the phone and wanted to show her the peacock pictures I posted on FF last night. Over here at the ranch the DSL is through a local ISP, small but excellent. I brought up FF here so I could talk her through finding the peacock-photos thread.
But she couldn't reach FF on that pc. She saw only an endless wait, not even a 404. Its on DSL provided by PacBell/Yahoo/ATT. I rechecked mine here and it worked fine, simultaneous with her difficulty. I would have thought that huge ATT's DNS servers would have caught up by now.