The disbelief was palpable when Mozilla’s CTO last month declared that AI-assisted vulnerability detection meant “ zero-days are numbered ” and “defenders finally have a chance to win, decisively.” After all, it looked like part of an all-too familiar pattern: Cherry pick a handful of impressive AI-achieved results, leave out any of the fine print that might paint a more nuanced picture, and let the hype train roll on. Mindful of the skepticism, Mozilla on Thursday provided a behind-the-scenes look into its use of Anthropic Mythos—an AI model for identifying software vulnerabilities—to ferret out 271 Firefox security flaws over two months. In a post , Mozilla engineers said the finally ready-for-prime-time breakthrough they achieved was primarily the result of two things: (1) improvement in the models themselves and (2) Mozilla’s development of a custom “ harness ” that supported Mythos as it analyzed Firefox source code.…