It does not work on the only website that I am interested into: archive.org.
Anyway I would never install it in my main browser, because it can see all my passwords and other things that I write.

See:

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."

Plus, once installed it changes its name to "Save video as", making it hard to find on addons.mozilla.org.