Mozilla Strengthens Firefox Security with Claude Mythos

Mozilla used Claude Mythos Preview to find and fix hundreds of security vulnerabilities in Firefox. In the past, AI-submitted security reports to open-source projects were mostly noise, but as models became more finely tuned and scaled up, and as the approach of layering multiple models to filter signal matured, actual usefulness increased significantly.

• Old defects such as a 20-year-old XSLT bug were discovered.
• A 15-year-old bug in the <legend> element was also among the fixes.
• Many attempts were blocked by Firefox's defense-in-depth layers.

The number of fixes stayed at around 20-30 per month throughout 2025, but surged in 2026 to 25 in January, 61 in February, 76 in March, and 423 in April. The monthly graph clearly shows this spike in the final month.