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Mercury vs Firefox ESR: catalog facts
Mercury	Firefox ESR
Free Free and open source.	Free Free and open source; Mozilla offers optional paid professional support for organisations.
Windows, macOS, Linux	Windows, macOS, Linux
Compiler-optimized Firefox builds Privacy patches drawn from LibreWolf, Waterfox and IceCat	Long-term stable feature set for managed deployments MSI and PKG installers plus group policy templates
Maintained by a single developer Updates can trail official Firefox releases	New features arrive later than in regular Firefox Mainly aimed at organisations rather than home users
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 23, 2026	Official website Full listing and sources Checked September 24, 2026

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