Some older add-ons (like those based on the XUL architecture) were discontinued after the "Quantum" update (version 57), and users may need an older build to keep them functional.
If you are looking to download an old version, you usually fall into one of three "Eras."
Always scan downloaded files with antivirus software.
Old Firefox lacks modern ciphers. Solution:
There are several scenarios where a legacy version of Firefox is preferable or even necessary: