On arm64 architectures, a vulnerability in the WebAssembly (WASM) br_table instruction with many entries could cause the branch label to be too distant from the instruction. This results in truncation and incorrect computation of the branch address, potentially leading to memory corruption. This flaw affects multiple versions of Firefox and Thunderbird prior to their respective fixed releases. The vulnerability has a critical severity rating with a CVSS score of 9.8. Mozilla has addressed this issue in Firefox 141, Firefox ESR 115.26, 128.13, 140.1, and corresponding Thunderbird versions.