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Firefox ESR vs Mercury: catalog facts
Firefox ESR	Mercury
Free Free and open source; Mozilla offers optional paid professional support for organisations.	Free Free and open source.
Windows, macOS, Linux	Windows, macOS, Linux
Long-term stable feature set for managed deployments MSI and PKG installers plus group policy templates	Compiler-optimized Firefox builds Privacy patches drawn from LibreWolf, Waterfox and IceCat
New features arrive later than in regular Firefox Mainly aimed at organisations rather than home users	Maintained by a single developer Updates can trail official Firefox releases
Downloadable app	Downloadable app
Open source	Open source
License: MPL-2.0	License: MPL-2.0
No account needed	No account needed
Not stated if it works offline	Not stated if it works offline
Official website Full listing and sources Checked September 24, 2026	Official website Full listing and sources Checked September 23, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.