Mozilla and Anthropic's Claude Mythos Uncover 271 Firefox Security Vulnerabilities Before Release

Mozilla shipped Firefox 150 with fixes for 271 security vulnerabilities surfaced by an early version of Anthropic's Claude Mythos Preview — the largest single-cycle total the browser team has recorded. It follows an earlier collaboration in which Claude Opus 4.6 led to 22 fixes in Firefox 148, a more than tenfold jump between model generations. Firefox CTO Bobby Holley described the disorientation of seeing that volume arrive at once, noting that against a hardened target a single bug of that kind would have been an emergency a year earlier. He also drew a more reassuring conclusion: nothing Mythos surfaced was beyond the reach of an elite human researcher, which suggests the model is finding what was previously missed rather than a new class of AI-exclusive flaw. Mozilla fixed 423 security bugs across its April releases in total. Mythos remains unreleased publicly, available only to a vetted group of organizations through Anthropic's Project Glasswing program.