It's not within the realm of believability. The chance of "human screwed up" astronomically exceeds this being fairly randomly chosen, and the Firefox bug states, (I think this is supposition, not known fact, but IDK)

> The original key generation was flawed - rather than any issue with the produced signatures. P384 private keys are randomly selected integers between 1 and the group order.

That is within the realm of believability.

When you have multiple selections you can show that the probability of selecting a set with a range much smaller than the population range is exponentially unlikely, strong evidence to reject the hypothesis that the subset was chosen at random. But when N=1 you can't conclude anything.

Regardless of the probability that this cosmically unlikely occurrence naturally happened 8 times in a row, we all know what the privkeys are now so the certs can't be trusted anymore.