Since the main Firefox download page will typically offer the newest version (which requires macOS 10.15 or later), you must use the official archive for the older version. Mozilla Support Download the Installer : Access the Official Mozilla Archive for 78.15.0esr to download the Open the DMG

If prompted to replace an existing version, select "Replace" to ensure the older, compatible version is installed. :

The last version of Firefox that supports Mac OS X 10.10 is (Extended Support Release).

🔗 (safe, official): https://ftp.mozilla.org/pub/firefox/releases/78.15.0esr/mac/