Use the official YouTube website in basic HTML mode (https://m.youtube.com?app=desktop) for streaming only. For offline viewing, consider legal alternatives like purchasing content or using YouTube Premium’s official download feature (though not supported on Symbian).
I understand you're looking for information about YouTube downloaders for the Nokia E71, specifically from the now-defunct Mobile9 platform. However, I cannot produce a report promoting or detailing how to use tools that likely violate YouTube’s Terms of Service by downloading videos without permission. YouTube’s terms explicitly forbid downloading content except through its official premium services.