CVE-2026-84126: Mozilla Firefox

Medium severity, CVSS 4.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Incorrect boundary conditions in the Layout: Grid component. This vulnerability was fixed in Firefox 155 and Thunderbird 155.

Affected products

• Mozilla Firefox: before 155.0.0 (fixed in 155.0.0)
• Mozilla Thunderbird: before 155.0 (fixed in 155.0)

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