Key Points

Mozilla applied an early version of Anthropic’s Claude Mythos Preview to Firefox as part of an ongoing collaboration.
The Firefox 150 release reportedly includes security fixes for 271 vulnerabilities that were identified during this initial evaluation.
Mozilla frames the effort as enabling defenders to act with greater decisiveness, suggesting improved focus and effectiveness from the testing work.
The post is presented as a quotation collected by Simon Willison rather than a full technical walkthrough or independent study.

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