CVE-2021-29984: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1.4% chance of exploitation in the next 30 days.

Instruction reordering resulted in a sequence of instructions that would cause an object to be incorrectly considered during garbage collection. This led to memory corruption and a potentially exploitable crash. This vulnerability affects Thunderbird < 78.13, Thunderbird < 91, Firefox ESR < 78.13, and Firefox < 91.

Affected products

• Mozilla Firefox: before 91.0 (fixed in 91.0)
• Mozilla Firefox ESR: before 78.13.0 (fixed in 78.13.0)
• Mozilla Thunderbird: before 78.13.0 (fixed in 78.13.0)

Published 2021-08-17. Last modified 2026-06-17.