Mozilla Hardens Firefox with Claude Mythos Preview, Finds 423 Bugs in a Single Month

Summary

Mozilla used early access to Claude Mythos Preview — Anthropic's new security-focused model — to dramatically scale Firefox vulnerability detection. The result: 423 bugs fixed in April 2026 alone, versus 20–30 per month throughout 2025. Specific vulnerabilities discovered include a 20-year-old XSLT bug and a 15-year-old HTML `<legend>` element vulnerability that had survived extensive prior fuzzing campaigns. Firefox's existing defense-in-depth architecture successfully blocked many exploit atte