Mozilla Uses Claude Mythos to Fix Hundreds of Firefox Vulnerabilities
Mozilla utilized the Claude Mythos preview to discover and address numerous vulnerabilities in Firefox. In the past, AI-generated bug reports were frequently regarded as subpar, but enhancements in model capabilities and better techniques for steering, scaling, and stacking models have changed this perception. This initiative revealed a 20-year-old XSLT bug and a 15-year-old issue with the <legend> element. Many attempted attacks were thwarted by Firefox's robust defense-in-depth strategies. While Mozilla was resolving approximately 20-30 security bugs monthly through 2025, this figure surged to 423 in April 2026. Simon Willison authored the post on 7th May 2026.
Key facts
- Mozilla used Claude Mythos preview to harden Firefox.
- AI-generated bug reports were previously considered low-quality slop.
- Model capability and harnessing techniques improved significantly.
- A 20-year-old XSLT bug and a 15-year-old <legend> bug were found.
- Firefox's defense-in-depth blocked many exploit attempts.
- Mozilla fixed 20-30 security bugs per month through 2025.
- In April 2026, they fixed 423 security bugs.
- The post is by Simon Willison, dated 7th May 2026.
Entities
Institutions
- Mozilla
- Firefox
- Claude Mythos
- Simon Willison