Another ongoing project called Serpent (based on Firefox 52-55 code) that backports modern TLS and web standards.
If you are looking to download the browser for a legacy system, the official Mozilla Archive is the most secure source. Firefox ESR 52.9.0, See All New Features, Updates and Fixes
Keeping an old Windows XP or Windows Vista machine running requires software that recognizes the OS architecture. Firefox 52.9.0 ESR provides the most stable, feature-complete browsing experience possible on those machines. 2. Accessing Legacy Enterprise Dashboards
For the millions of machines still running Windows XP (e.g., point-of-sale systems, industrial control panels, museum kiosks, and classic gaming rigs), Firefox 52.9.0 ESR represents the final secure gateway to the modern web. After this version, no further Firefox updates—security or otherwise—would work on those systems.
Keep the computer off the general internet if possible. Use it strictly on local intranet networks.
It lacks modern sandboxing techniques and tracking protection mechanisms found in current browser versions. Mitigation Strategies
https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/en-US/Firefox%20Setup%2052.9.0esr.exe
Last updated: October 2025
Finding a legitimate download for Firefox 52.9.0 ESR requires using Mozilla’s official archives to avoid malware-laden third-party sites. Users should navigate to the Mozilla FTP or "Directory" servers. From there, you can locate the 52.9.0esr folder and select the appropriate sub-folder for your operating system (win32, win64, or mac) and your preferred language.
Beyond OS compatibility, 52.9.0 ESR was significant for what it preserved—and what it transitioned away from: NPAPI Plugins:
the Firefox Setup 52.9.0esr.exe from the FTP link provided above.
If your hardware allows it, the best and safest path is to upgrade your operating system to a modern, supported version (like Windows 10 or 11) and use the latest version of Firefox. This will protect your data and provide you with the best possible web experience.
If you downloaded it for a specific Java applet, keep Flash and Silverlight disabled in the settings menu to reduce the attack surface. Modern Alternatives for Older Operating Systems