> In other words up to 10% of all the crashes Firefox users see are not software bugs, they're caused by hardware defects! If I subtract crashes that are caused by resource exhaustion (such as out-of-memory crashes) this number goes up to around 15%.

Crashes caused by resource exhaustion are still software bugs in Firefox. At least on sane operating systems where memory isn't over-comitted.

What's the expected behavior of a JavaScript program that allocates all memory on the machine?

Memory isn't the only resource.