Once you've downloaded an older version of Firefox:
Before downloading an older version of Firefox: download firefox older versions
If you download and run Firefox version 56.0 today: Once you've downloaded an older version of Firefox:
the executable file (typically .exe for Windows or .dmg for Mac). Why Users Download Older Versions Install an older version of Firefox - Mozilla Support While these are generally safe, they are not
Whether you are a developer testing legacy web applications, a retro computing enthusiast trying to breathe life into an old operating system, or simply someone who prefers the interface of a bygone era, there are valid reasons to seek out older versions of Mozilla Firefox.
Sites like the Internet Archive (Wayback Machine) or reputable software history sites often host installers. While these are generally safe, they are not official sources. Always verify the file hash if possible.