Run the installer. You may be prompted to create a new Firefox profile because older versions often cannot read data from newer profiles. Critical Precautions and Risks Using an outdated browser comes with significant warnings:
[ Firefox 56.0.2 ] | Oct 9, 2017 • Last version with full XUL/legacy add-on support • ⚠️ 12 known security issues • Windows 7+ / macOS 10.9+ / Linux [ Download (52 MB) ] [ SHA256 ▼ ]
Downgrading can sometimes corrupt your profile. Back up your bookmarks and passwords first.
As a user who needs compatibility with legacy extensions, enterprise environments, or older operating systems, I want to browse and download older versions of Firefox so that I can install a specific version that meets my technical requirements.