firefox used to freeze my comp too, this was on XP
Not experienced any problems on Vista x64 yet
From my experience forget overheating, its most likely not it, you may have a corrupt firefox profile somehow or a faulty piece of RAM (unfortunately very common)
...Then there is also drivers and general compatibility to consider also, most importantly motherboard and graphics card stuff with regards to system locks, restarts and failures.
hope this is of some help
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