It's mind blowing for me whenever the hardware abstraction leaks like this. You (ie Firefox) had to call the instructions in a specific order to cause the hardware to fail.

It does make sense that a browser, being the thing most people would be running all day, basically fuzzes CPUs for free... they generate their own CPU instructions at runtime from the JIT engine(s), and different styles of webpages could cause every possible combination of instructions to appear throughout the day on every machine.