Key Points

Mozilla says Firefox 150 includes fixes for 271 vulnerabilities discovered during an early evaluation of an Anthropic Claude Mythos Preview used in the Firefox context.
The CTO frames the collaboration and evaluation as a turning point for defenders, suggesting AI-enabled security work can help systematically address zero-day classes.
The post highlights the operational challenge of adopting AI-assisted workflows but emphasizes the resulting momentum and focus within the security team.
This quotation is presented as part of a continuing collaboration between Mozilla and Anthropic rather than a standalone new product release.

Simon Willison’s Weblog

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22nd April 2026

As part of our continued collaboration with Anthropic, we had the opportunity to apply an early version of Claude Mythos Preview to Firefox. This week’s release of Firefox 150 includes fixes for 271 vulnerabilities identified during this initial evaluation. [...]

Our experience is a hopeful one for teams who shake off the vertigo and get to work. You may need to reprioritize everything else to bring relentless and single-minded focus to the task, but there is light at the end of the tunnel. We are extremely proud of how our team rose to meet this challenge, and others will too. Our work isn’t finished, but we’ve turned the corner and can glimpse a future much better than just keeping up. Defenders finally have a chance to win, decisively.

— Bobby Holley, CTO, Firefox

Posted 22nd April 2026 at 5:40 am