If you are downloading for a corporate environment, avoid standard version 52.0. You want .
Steps to obtain an old Firefox build safely Download Firefox 12-51 52 Esr
While newer versions like Firefox Quantum offer superior speed and security, these specific older versions are unique for several reasons: If you are downloading for a corporate environment,
and select "Check for updates but let you choose whether to install them." Directory Listing: /pub/firefox/releases/ - mozilla
If you are downgrading, you may need to create a new profile if prompted, as newer profiles are often incompatible with older browser versions. Directory Listing: /pub/firefox/releases/ - mozilla.org
Understanding the Legacy: Firefox Versions 12–51 and 52 ESR
: Once installed, immediately go to Options/Preferences > General > Firefox Updates and select "Check for updates but let you choose to install them" to prevent the browser from automatically updating to the latest (incompatible) version.