Firefox reports a massive April spike in security fixes after using Claude Mythos for bug hunting
Summary
Mozilla reports a significant increase in security fixes for Firefox in April after utilizing Claude Mythos to assist in bug hunting and hardening the browser.
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Mozilla used the Claude Mythos preview to systematically find and fix hundreds of security vulnerabilities in Firefox, dramatically increasing their bug-fix rate from around 20-30 per month to 423 in April 2026.
Hardening Firefox with Claude Mythos Preview
Mozilla details how they used Claude Mythos Preview and other AI models to identify and fix a significant number of latent security bugs in Firefox, demonstrating a shift in the efficacy of AI for code hardening.
Mozilla Used Anthropic’s Mythos to Find and Fix 271 Bugs in Firefox
Mozilla used Anthropic’s Mythos Preview to automatically identify and fix 271 vulnerabilities in Firefox 150, signaling a major shift in how software security will be managed as AI-powered bug discovery becomes widespread.
Quoting Bobby Holley
Firefox 150 shipped with 271 security fixes found by Anthropic’s Claude Mythos Preview, marking a major AI-driven win for defensive security.
@FinanceYF5: With the help of Claude Mythos Preview, the Firefox team fixed more security vulnerabilities in April than in the previous 15 months combined.
Powered by Claude Mythos Preview, the Firefox team fixed more security vulnerabilities in April than in the past 15 months combined, demonstrating AI's significant role in boosting the efficiency of software security maintenance.