CVE-2025-8027 is a vulnerability in the IonMonkey JavaScript JIT compiler used by Firefox and Thunderbird on 64-bit platforms. The issue arises because IonMonkey-JIT only writes 32 bits of the 64-bit return value space on the stack, while Baseline-JIT reads the full 64 bits, leading to inconsistent behavior. This flaw affects multiple versions of Firefox and Thunderbird prior to their respective fixed releases. The vulnerability has a CVSS score of 6.5 and is considered medium severity. Mozilla has released fixes in Firefox 141 and Firefox ESR 115.26, 128.13, and 140.1, as well as corresponding Thunderbird versions. No known exploits in the wild have been reported.