A race condition vulnerability in Firefox and Thunderbird could cause private browsing tabs to open in normal browsing windows, potentially leading to a privacy leak. This issue affects Firefox versions prior to 135, Firefox ESR versions prior to 128.7, and Thunderbird versions prior to 128.7 and 135. The vulnerability has a medium severity rating with a CVSS score of 6.5. Mozilla has fixed this issue in Firefox 135 and Thunderbird 135, as well as Firefox ESR 128.7 and Thunderbird 128.7.