If you are looking for a slightly older version of the modern browser—perhaps because a recent update broke a feature or isn't compatible with your hardware—you can find archived installers on several reputable mirrors.
The Digital Time Capsule: Why Users Still Cling to Old Versions of Waterfox
If you are looking for an old version simply to avoid UI changes, try the latest version of Waterfox first. It offers extensive customization options via about:config that can often mimic the feel of older browsers without sacrificing security.
Specifically, the search for the has become a surprisingly common trend among power users, enterprise administrators, and privacy enthusiasts. Why are people actively seeking outdated builds? Is it safe? And which specific old version should you download?