Only a handful of users have access to Anthropic's Mythos tool. Mozilla is one, and it says it found hundreds of security bug fixes. Samuel Boivin/NurPhoto via Getty Images Mozilla is one of the companies that has access to Anthropic's powerful new Mythos model. The company said it fixed 423 security bugs in April. That's 398 more than it did in January. Previous AI systems produced "slop," Mozilla wrote. AI is getting scary good at finding software flaws, according to one company that got early access to Anthropic's powerful new Mythos model. On Thursday, Mozilla, the nonprofit behind the Firefox web browser, posted a blog detailing how it used Claude Mythos Preview — an unreleased AI model from Anthropic — to uncover security issues. The software company said the AI model discovered hundreds of issues, including at least one that had survived for 20 years. Mozilla detailed 12 bugs it fixed. One, it said, went "undetected for years by fuzzers," the security-testing tools that try to break software.…