Try different download software. Try Internet Download Manager 6.21 registered. It can download videos from not only YouTube but also from many websites having videos, sound, software, documents, compressed files and others. It will work.

Please report back to say if this helped you!

Thank you.

Youtube made site changes which they do every so often. The Video DownloadHelper Extension author has pushed a update due to Youtube changes.

As you can see. https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/ Yesterday, YouTube changed their page structure causing Video DownloadHelper not to detect any video. A fix has been pushed to Mozilla servers as version 4.9.23.

As recommended: Uploaded and reinstalled Download Helper. Uploaded and reinstalled updated Flash Player

Restarted computer and still not working

And for the Mac, what alternative?