This extension actually works, however, It is almost impossible to locate the real downloading URL location/button because the author has liberally added adverts which look as though they would be the download URL for this (good) firefox add-on. For instance, I just received an email touting an improvement to this add-on, but was unable to locate the download url to it.
UPDATE: improvements no longer have to be downloaded, since they automatically are installed by this extension.
If you can find the URL for the Author's support email, you will actually get a response in about 3 days. My experience is that if you don't give up after receiving the first, generalized response, you will eventually be very satisfied by the author's support, e.g., it took me 4-5 emails to get exactly the information I was seeking.

This addon worked for me all the time. Great content provided from the developers, thank you very much!

Amazing

It works, even when YouTube changes his code.

Youtube Video Downloader is an outstanding extension for 2 reasons:
(1) it provides an important functionality and it does it very well. Providing actual video files from YT is not a trivial convenience, it helps me do my work (I am a researcher and physician)
(2) the extension is very difficult to maintain, because of the incessant changes on the YT platform. That is why I strongly recommend everyone financially support this extension who can afford to do so.

I love this addon and the developer always works to keep it updated as quick as possible when changes in youtube occur.

my man be workin

This is a great product in its category, the best & fastest I've found over the years. The only request I'd make is for it to equally work on sites other than YT.

Excellent add-on!

Does what it says, great add-on!

Excellent add-on! Lots of work goes into it and Google's ever-changing practices, I'm sure :)

This is a great add-on. It goes down periodically, but this is the only such add-on that is persistently & effectively maintained in the face of frequent updates & changes to the way the internet works.

The developer is working on a fix

Excellent app. Developer works hard to keep it up to date when YouTube changes their codes. Sometimes there's a short lag for the update, but that's to be expected. Thanks for a great program!

Clearly the best video download extension. It's updated quickly every time YouTube tries to make downloading more difficult. Works better and on more video hosting platforms than other extensions.

Works really well! Love it!