I don’t think they’re arguing that Firefox is more susceptible to bit flips. They’re trying to say that their software is “solid” enough that a significant number of the reported crashes are due to faulty hardware, which is essentially out of their control.

If other software used the same methodology, you could probably use the numbers to statistically compare how “solid” the code base is between the two programs. For example, if the other software found that 20% of their crashes were caused by bit flips, you could reasonably assume that the other software is built better because a smaller portion of their crashes is within their control.