Some other possible reasons: 1) Mozilla as an organization has previously expressed the opinion that contests like this have bad side-effects (like causing de

Some other possible reasons: 1) Mozilla as an organization has previously expressed the opinion that contests like this have bad side-effects (like causing delays in vulnerabilities being reported by researchers who find them, hence leaving users exposed when the vulnerability could have been fixed). Keep in mind that the browser vendors are involved in funding the contest, so said opinion could be read as "we don't want to pay you money for something we think is harmful". 2) Mozilla has had the best response times of the browser vendors on Pwn2Own bugs, which means the other vendors have incentive to keep them out of the contest if they can spin it as a problem with Firefox, not with the contest. Notably, Firefox has made security improvements in the last year. "Serious", is of course a subjective judgement.