I've seen a number of threads about Video Download Helper not working right. I've been using this: https://addons.mozilla.org/en-US/firefox/addon/download-youtube/

Granted it just works with YouTube.

Basically, just have to wait for the developer to fix their plugin.

Installed your addon but the button did not display in firefox so cannot test it. I don't think the problem is download helper I think the download function in firefox is broke.

Go to the programs support site and ask questions.

There is a new version of the Video DownloadHelper available here. https://groups.google.com/forum/#!forum/video-downloadhelper-5

It may be a few weeks before it is available from the Mozilla Addons website, as it is a complete rewrite of that addon.