Please consider turning on send info to Mozilla before posting a question. This helps us help you. As I have no info from your system that would help see things that cause issues. I would have seen this; what Youtube Downloader you had : but now I ask what Youtube Downloader you have as you do not say ?

Go to your Extensions page copy/paste about:addons into the Address Bar then Enter. You will be in your Extensions Manager. Click find A Replacement as there are many, and/or https://www.google.ca/search?q=firefox+Youtube+Downloader&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&dcr=0&ei=dE4OWpT6Lo-D-gS7nYSYAQ Your missing Extensions have/had been marked for many months as Legacy and some info.

As for your videos you will have to look through your hard drive for where they were placed by unknown Youtube Downloader Can try in C:\User and check the folders or find the Developers Page and see if there was a FAQ or more info.

How about looking at these 2 URL's first : https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/ and The name is not descriptive enough : https://www.howtogeek.com/332449/whats-new-in-firefox-quantum/

More information will come out slowly. New extensions are being made and some older ones being updated that can track here : http://bit.ly/2j0Anlj

Please let us know if this solved your issue or if need further assistance.

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This download manager works separately from the browser. The browser tells it what to download, and off it goes.

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