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Install an older version of Firefox | Firefox Help - Mozilla Support
Directory Listing: /pub/firefox/releases/ - Mozilla
Download Old Versions of Mozilla Firefox for Windows - OldVersion.com
Installing a previous version of Firefox | Mozilla Support
Where can I get an earlier version of Firefox (V51 or earlier)?
Directory Listing: /pub/firefox/releases/80.0/ - Mozilla
How to downgrade/download to older versions of Firefox
Firefox version history - Wikipedia
Where can I find really old versions of Firefox? - Super User
Directory Listing: /pub/firefox/releases/78.0/ - Mozilla