CVE-2025-1010 is a critical use-after-free vulnerability in Mozilla Firefox and Thunderbird affecting versions prior to Firefox 135 and Firefox ESR 115.20/128.7, as well as Thunderbird versions prior to 128.7 and 135. The flaw exists in the Custom Highlight API and can lead to a potentially exploitable crash. Mozilla has released fixes in Firefox 135 and corresponding Thunderbird and ESR versions to address this and related memory safety issues.