A: YouTube will work in basic HTML5 mode (480p/720p). Netflix will likely fail because Widevine DRM on Firefox 78 is too old to validate current licenses.
A: The modern Firefox codebase uses APIs (Application Programming Interfaces) from macOS 10.12 Sierra and above. Modifying the binary would break core rendering. firefox download for mac 10105 upd
While this setup is not recommended for primary, secure daily driving, it is the best possible modern browsing experience available for your legacy hardware. Keep your expectations realistic, use a robust ad-blocker, and consider upgrading your machine at the earliest opportunity. A: YouTube will work in basic HTML5 mode (480p/720p)