Arrgh, I'm at work right now and can't see images posted in the forums
But it shouldn't be a heat issue if you're just running Firefox, especially after a clean install (there won't be that many convoluted processes running). Do you have extra parts that you can start the swap process with? Replacing the HD, GPU, RAM (although I highly doubt it's a RAM issue) will at least get you going down the right path of troubleshooting. Random lockups without a powerdown typically can't be the PSU, even though I generally lean towards that as these fail very easily. Try testing a different PSU as well. You can also check to see if any of your hardware was assigned the same I/O address which would also cause random freezes like this. However, a fresh install should have corrected that issue as well :/
Let us know what, if anything, you find after playing Dr. with different parts.
Humans are the only creatures that won't live up to the their potential. Give everything your all. When you die, die on E.
So I ask you, what do you intend to do now?