CVE-2026-84134: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 0.6% chance of exploitation in the next 30 days.

Other issue in the Profile Backup component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

Affected products

• Mozilla Firefox: before 153.2.0 (fixed in 153.2.0); from 154.0.0, before 155.0.0 (fixed in 155.0.0)
• Mozilla Thunderbird: before 153.2.0 (fixed in 153.2.0); from 154.0, before 155.0 (fixed in 155.0)

Published 2026-09-01. Last modified 2026-09-03.