Twice this morning out of the blue the entire machine has shut down. The first time my son had been playing some online games using Firefox. He'd been on for maybe an hour or two and he comes running in and tells me the entire machine shut down...not went to sleep, shut down. I waited a couple of minutes and turned the machine back on. It hadn't been running more than 30 or 45 minutes. This time my wife is on the machine and again, I think she was using FireFox checking Facebook. I was in the kitchen heating up some lunch when for a brief second the power to the microwave seemed to fluctuate. I heard my wife from my office say the computer had just shut down. Again, I waited for about 5 minutes and turned the machine back on. I keep using my iStat widget to see if the fans are excessively spinning or the temps are overheating but everything is normal. My first thought has been that the machine is shutting itself down to overheating. Any ideas on what's happening? The only commonality between the situations is that both people we using FireFox at the time but I've used it countless times and a complete shutdown has never happened to me. Should I call this one into AppleCare on Monday?