I have several hobbies and use tutorials to continue learning about then. I have used Video DownloadHelper for years to help me download the best of them. It downloads seemingly every format of video, although with the newer version of Firefox it needs an extra bit of separate code to help it achieve it, but that's not a problem. I haven't yet found another addon that can do the same.
So now I have a video library of all my most useful tutorials, thanks to Video DownloadHelper. Highly recommended!
P.S. It does lot of other things as well.

working as of 14 Feb 2018

A large number of settings and the ability to download the file in high resolution. Cool.

no problems at all.

I like Video DownloadHelper because it is easy to use and downloads very quickly. Once I figured out how to use it, I found it to be a very efficient tool. I recommend it to Mozilla users.

It's easy, safe and does the trick. It can download almost every video that you can play on your computer from anywhere.

A melhor extensão do mundo para baixar vídeos e qualquer arquivo de mídia em qualquer site.

Très pratique, détecte automatiquement les vidéos et met en pause les téléchargements successifs pour éviter d'en avoir trop en même temps.

Llevo semanas sin poder instalar esta aplicación, he probado a revisar el ordenador, y no hay manera, dice que la instala, peor no te deja hacer ABSOLUTAMENTE NADA, de hecho ni siquiera aparece el icono de los globos para poder usarla.
Entrando en el menu complementos te dice que está instalada, pero no no te da la opción a abrirla.
La desinstalo para intentarlo de nuevo , pero el problema se repite sin parar

Incrivel esse addons, recomendo adquirir, simplesmente, maravilhoso

Doesn't work at all . My phone Oppo F5

很好 非常不错

Me parece un anténtico timo, ya que no funciona ni medio bien (anteriormente funcionaba de forma correcta) y solo descarga pequeños archivos que no valen para nada. La mejor opción es desinstalar el complemento

It's good.
I love the fact that I'm not forced to install companion app directly to my system - I can do it in my user-space.

Nice app

Even the optional addon to download is a simple install with no hassle, adware or virus crap. Makes it a hell of a lot easier. Thanks guys for taking the high road and not selling out.

Muy practico y versatil para descargar y convertir videos de la web.

me permite descargar solo que aveces es medio complicado me gustaria fuera mas sencillo

works on most sites.