Anthropic’s Mythos redefines Firefox’s cybersecurity approach

When Anthropic introduced its Mythos model in April, the company also issued a strong caution to software developers. The model proved so adept at identifying software vulnerabilities, the lab claimed, that it uncovered thousands of high-severity bugs that needed patching before the model could be released publicly.

Now, security researchers working on Mozilla’s Firefox browser are offering a detailed look at how that process has played out in practice and what Mythos’ capabilities mean for software security more broadly.

In a post published Thursday, Mozilla stated that Mythos has revealed a large number of high-severity flaws, including some that had remained hidden in the code for over a decade.

That marks a major leap forward from what AI security tools could achieve just six months earlier. Until recently, AI-powered bug-finding tools came with serious limitations, often overwhelming security teams with low-quality reports and false alarms. However, Mozilla’s researchers say the latest generation of tools has reached a turning point, especially now that agentic systems can evaluate their own work and filter out poor results.

“It is difficult to overstate how much this dynamic changed for us over a few short months,” the researchers wrote. “First, the models got a lot more capable. Second, we dramatically improved our techniques for harnessing these models.”

Image Credits:Firefox

The results are striking: In April 2026, Firefox shipped 423 bug fixes, compared to just 31 in the same month a year earlier. The researchers have also shared details on 12 of those bugs, ranging from two unusual sandbox vulnerabilities to a 15-year-old error in how the browser parses an HTML element.

“These things are actually just suddenly very good,” Brian Grinstead, a distinguished engineer at Mozilla, told TechCrunch. “We see that on our own internal scanning, we see that on external bug reports, and we see that in all sorts of signals across the industry.”

The fact that the system helped uncover vulnerabilities in Firefox’s “sandbox” system is especially noteworthy, given the complexity required for an exploit targeting it. To find sandbox vulnerabilities, the model must craft a compromised patch for the browser, then attack the most secure part of the software with the new code in place. Finding and demonstrating the bug is a delicate, multi-step process that demands both creativity and precision.

To put this into perspective, Mozilla’s bug bounty program offers researchers up to $20,000 for discovering a bug in Firefox’s sandbox — the highest reward available. Despite that top-dollar bounty, Grinstead says Mythos is finding more sandbox issues than human researchers ever did. “We do get them,” he told TechCrunch, “but not at the volume that we are able to find with this technique.”

Notably, the Firefox team still isn’t using AI to fix the bugs, despite well-documented progress in AI coding tools. The team does ask AI to write patches for each bug, but the resulting code usually can’t be deployed directly and instead serves as a reference for a human engineer.

“For the bugs we’re talking about in this post, every single one is one engineer writing a patch and one engineer reviewing it,” Grinstead says. “We have not found it to be automatable.”

It remains unclear how AI’s emerging capabilities will reshape the broader cybersecurity landscape. One month after Mythos was previewed, most of the bugs it discovered likely haven’t been patched yet, making it hard to gauge their full impact. Anthropic has been meticulous about following responsible disclosure norms, but it’s probable that malicious actors are using similar techniques behind the scenes, even if the models they employ are less advanced.

Speaking at a recent event, Anthropic CEO Dario Amodei expressed optimism that the new tools would ultimately favor defenders. “If we handle this right, we could be in a better position than we started, because we fixed all these bugs. There are only so many bugs to find,” Amodei said. “So I think there’s a better world on the other side of this.”

Having dealt with the practical realities, Grinstead offers a more tempered view: “It’s useful for both attackers and defenders, but having the tool available shifts the advantage a little bit to defense. Realistically, nobody knows the answer to this yet.”