Seriously ????!!!!!!
The screenshot you showed does not display anything coming from our add-on. Not convinced ? Go to the browser add-ons tab and disable Video DownloadHelper, then reload your page and you will see the exact same thing. For information, Video DownloadHelper does not write into YouTube page to add buttons, it's all from the toolbar button.
If i'm right i get my donation ? :)
And by the way, if you have problems downloading from YouTube, just give a try to Video DownloadHelper, it works well !