Mozilla AI Tool Finds Hundreds of Firefox Security Flaws

Mozilla reported that the Mythos AI tool from Anthropic uncovered 271 security vulnerabilities in the Firefox web browser ^[1].

This development marks a significant shift in how software companies identify security gaps. By utilizing artificial intelligence to scan codebases, developers can potentially find critical flaws faster than human auditors while reducing the time spent filtering out incorrect reports.

The bug hunt took place in April 2026 ^[2]. While one report states that 271 vulnerabilities were found ^[1], another source indicates that Mozilla identified 423 Firefox security bugs during the same period using the tool ^[2].

Mozilla said that the process resulted in almost no false positives ^[1]. This level of accuracy is often a primary hurdle for automated security tools, which typically generate a high volume of "noise" or incorrect alerts that engineers must manually verify.

To achieve this precision, Mozilla used a harness-guided Mythos analysis combined with a second large language model to verify the findings ^[1]. This multi-stage verification process ensured that the reported flaws were legitimate before they reached the security team.

Grinstead, a Mozilla security lead, said the details provided by the analysis and confirmed by the second LLM provide a level of confidence his team didn't have before ^[1]. He said that this approach differs from previous vulnerability disclosure "slop" that often plagues automated reports ^[1].

The integration of Mythos into the Firefox codebase allows the security team to focus on remediation rather than validation. By automating the discovery of flaws with high confidence, Mozilla can patch vulnerabilities before they are exploited by malicious actors.