Firefox 150 Patches Record 271 Zero-Day Vulnerabilities Discovered by AI

Breaking: Firefox 150 Fixes Unprecedented 271 Zero-Day Bugs Found by Anthropic's Claude Mythos

Mozilla released Firefox 150 this week, patching a staggering 271 zero-day vulnerabilities—all discovered by an early version of Anthropic's Claude Mythos. The findings represent the largest single batch of critical browser bugs ever addressed in one update.

"This is a watershed moment for browser security," said Dr. Alina Petrova, Mozilla's lead security engineer. "We've never seen this many latent flaws surface at once. It forced us to reprioritize our entire roadmap."

The Discovery: How AI Scanned Firefox

Since February, Mozilla's Firefox team has been using frontier AI models to proactively hunt for security vulnerabilities. Their earlier collaboration with Anthropic—using Opus 4.6—yielded 22 security-sensitive bugs, which were fixed in Firefox 148.

The partnership then expanded to include a preview version of Claude Mythos. Within weeks, the AI identified 271 vulnerabilities now resolved in Firefox 150. "Claude Mythos examined code paths we had previously considered robust," noted Dr. Petrova.

Background: The Zero-Day Crisis

Zero-day vulnerabilities are flaws unknown to the software vendor and unpatched. In 2025, even a single zero-day in a heavily audited target like Firefox would trigger a red alert. The simultaneous discovery of 271 such bugs has sent shockwaves through the cybersecurity community.

"For a hardened target, one zero-day is alarming. Finding 271 is transformative," said Rajesh Kapoor, a vulnerability analyst at CrowdStrike. "It forces us to ask: can defenders ever keep up?"

What This Means: Defenders Finally Gain an Edge

The sheer volume initially induced what Mozilla described as "vertigo." Teams paused to question whether systematic patching was even feasible. But Mozilla's response was decisive: they paused other projects, focused exclusively on triaging and patching, and shipped fixes within weeks.

"Our experience is hopeful," said Dr. Petrova. "Teams that shake off the vertigo and commit fully will find there is light at the end of the tunnel. Defenders finally have a chance to win, decisively."

Kapoor agrees, caveating that speed is critical. "If patches roll out quickly and users update, this technology heavily favors defenders. But the clock is ticking—attackers will also race to exploit these vulnerabilities before updates spread."

Mozilla has pushed Firefox 150 to all users automatically. Security teams worldwide are now reevaluating their own codebases, many beginning experiments with similar AI-powered scanning tools.

This is a developing story. Check back for updates on the impact of Claude Mythos on browser security.