This is by far the best video downloader addon in Firefox, in my opinion.

But it also tricks you into paying for the companion app. Why do I say "tricks" - because it doesn't say in its description or before installing the companion app, that you will have to pay or else there will be a BIG watermark on the converted videos. You will need to use the convert feature and the companion app to download 1080 or higher resolution videos.

I used to be a fan of this plugin, unfortunately not anymore. This plugin stays installed, untill a good alternative is found (which I thinks is going to be hard to find!)