CVE-2025-1012 is a use-after-free vulnerability caused by a race condition during concurrent delazification in Firefox and Thunderbird. This flaw could lead to memory corruption and potentially exploitable crashes. It affects versions of Firefox prior to 135, Firefox ESR prior to 115.20 and 128.7, and Thunderbird prior to 128.7 and 135. Mozilla has fixed this issue in Firefox 135 and corresponding Thunderbird and ESR releases.