CVE-2017-5428 aka MFSA-2017-08 is an integer overflow vulnerability in Mozilla Firefox. It was reported through the Pwn2Own contest, and corrected in Firefox 52.0.1.
This article describes it as a zero-day vulnerability, but I thought Pwn2Own used a coordinated disclosure policy? The Mozilla bug isn't open yet, and I can't find exploit information anywhere else, but perhaps I just don't know where to look.
Do we need to panic (so to speak) or can we treat 52.0.1 on the same urgent-but-don't-panic basis as every other web browser security update?