The addon Video Downloader is currently hijacking links. It will convert some links to some smartdirect.de/xxx links.
Its behavior is just like this post on reddit.
Five reviews of this add-on have also reported this problem. Three users noticed this problem two months ago:https://addons.mozilla.org/en-US/firefox/addon/vid-downloader/reviews/?score=1
However, this add-on is still available now.