Since I am rating it low, I will explain.

Sometimes it does not download the video.
For example the following does not work:
https://www.youtube.com/watch?v=LPxIssabhTc

Sometimes it downloads at 360p, although the video is playing at 1080.
Example:
https://www.youtube.com/watch?v=sIaT8Jl2zpI

As the following page states:

https://support.mozilla.org/en-US/kb/permission-request-messages-firefox-extensions#w_access-your-data-for-all-websites

"The extension can read the content of any web page you visit as well as data you enter into those web pages, such as usernames and passwords."