Firefox 121 End of Life Date
Firefox 121 end-of-life date, support status, and CVE risk. Data from endoflife.date and official vendor documentation.
Firefox 121 was released on December 19, 2023 and support ended on January 23, 2024 — a supported lifespan of 35 days, longer than the 28 days median for Firefox releases. It was not an LTS release; Firefox reserves longer patch windows for its LTS versions. The last patch it will ever receive is 121.0.1, published January 9, 2024 — 14 days before support ended. It has been without security patches for 2 years and 7 months. 31 newer Firefox releases have shipped since.
What does Firefox 121 end of life mean?
When Firefox 121 reaches end of life, the maintainers stop issuing security patches for this version. CVEs discovered after the EOL date are publicly disclosed on the National Vulnerability Database with no patch available. Exploit code frequently appears on GitHub within days of disclosure.
The CVE blind spot: Most vulnerability scanners check for known CVEs but do not flag the ongoing accumulation of unpatched vulnerabilities in EOL software versions. Running Firefox 121 past its EOL date creates a permanently growing attack surface that standard security tooling will not surface.
Migrate to Firefox 153 or implement compensating controls — network segmentation, enhanced monitoring, restricted access — while migration is underway.