LoL if the Firefox causes this problem then I can surely tell you that this is not because of the Firefox.I mean I used Firefox many times and had many many tabs opened and when I would close them all at once,there wouldn't be any crash.My point is that it can be a virus that crashes your computer at a specified time,bad process,terminated process that was not supposed to be terminated,bad registry or completely corrupted operating system files.But it is SURELY not the Firefox lol.
You said it is not overheating...well it can be that too.Your temperature grews depending on the tasks you are performing.Maybe today it crashed only once because you were doing something simple such as the internet surfing and the other day it crashed 5 times while you were making a big video editing and some other big tasks which take a lot CPU usage.Check the temperature in the BIOS and let us know.
Cheers!