Half-Good, download some.

When it's available, it works like a dream, but it often vanishes from YouTube without warning for long periods. I have no idea why.

its a little bit scratchy

The addon doesn't work well with the new YouTube layouts and can't download to 1080p.

This Add-On , and Adblocker Ultimate are ABSOLUTLY the GREATEST !!! , Can't say enough good things about them BOTH , 100% Recommended by Mannta Aeronautics !!!

Pulled the trigger on Pro. I'm on the fence. I get blocked IP a lot. I wish people would give up the ghost on doing this server side and do what another add-on did and just download both files locally and use a client app to encode them. Or just give us the option to download ourselves and I'll do it myself.

Paid PRO version currently doesn't work with Firefox v79.0. Worked perfectly for years prior to a few days ago. Working with Support to try to determine what is causing download process to stall. Will submit updated review when (if?) solution is found.

Suele funcionar bien por un tiempo, pero cada vez que se actualiza Firefox presenta fallos y es necesario desinstalarlo y esperar a que actualicen también el add-on.
Adicionalmente, suele presentar algunos errores al bajar videos en 720px. Algunas veces muestra descarga, pero al bajarla resulta que esa versión no existía y lo que hace es bajar el último video que hayas descargado a 360px y sobre escribir el que crees que estás bajando a 720px.
Sin em bargo, es una buena extensión vale la pena probarla y utilizarla.