If we don't know what the reason was, and they won't elaborate on it, what importance does it have? Pwn2Own 2016 already happened. Firefox wasn't included, for whatever reason. The damage is already done at this point, as its exclusion created the
impression that it was "not worth attacking".
If the guys had said, "We didn't bother with Firefox because they weren't willing to pay us as much as Google or Microsoft", okay. But they didn't. What they said was:
'We wanted to focus on the browsers that have made serious security improvements in the last year,' Brian Gorenc, manager of Vulnerability Research at HPE said."
And now Firefox looks weak by comparison.