Statement from the NDSS 2021 Program Committee: NDSS is devoted to ethical principles and encourages the research community to ensure its work protects the privacy, security, and safety of users and others involved. While the NDSS 2021 PC appreciated the technical contributions of this paper, it was the subject of a debate in our community regarding the responsible disclosure of vulnerabilities for the Firefox web browser. The PC examined and discussed the ethics concerns raised and the authors' response. Although no harm came to users, the authors' oversight could have made a non-vulnerable browser vulnerable to the attack proposed in the paper. The PC does not believe the authors acted in bad faith. Nevertheless, we decided to add this note as well as the authors' response (in an Appendix) to the paper because the NDSS PC takes both the ethics of responsible disclosure and fairness towards the authors seriously. It is the PC's view that researchers must not engage in disclosure practices that subject users to an appreciable risk of substantial harm. NDSS will work with other conferences to further improve the transparency of vulnerability disclosure to reduce such errors in the future.