Because this is a legacy version, you cannot download it from the standard Firefox homepage. You must access Mozilla's official archive:
Firefox 78 was the final "major" release to support macOS 10.9, 10.10, and 10.11. To extend its life, Mozilla moved these users to the channel, which provided security patches until October 5, 2021. firefox mac 10.10 5 download
The latest and final version of Firefox compatible with is Firefox 78.15.0esr . How to Download Firefox for Mac 10.10.5 Because this is a legacy version, you cannot
If you are running and need a functional web browser, you have likely found that modern versions of Firefox no longer support your operating system. The latest and final version of Firefox compatible
: Visit the Mozilla FTP Archive to download the .dmg installer for version 78.15.0esr .
: Drag the Firefox icon into your Applications folder.