Enhanced Tracking Protection's Strict mode may have inadvertently allowed a CSP `frame-src` bypass and DOM-based XSS through the Google SafeFrame shim in the Web Compatibility extension. This issue could have exposed users to malicious frames masquerading as legitimate content. This vulnerability affects Firefox < 133, Firefox ESR < 128.5, Firefox ESR < 115.18, Thunderbird < 133, and Thunderbird < 128.5.