It seems to give different options for the video quality each time you try to download the same thing.
When you just click on a download option it doesn't ask you where you want to save or tell you where it's saving it, you have to click on the right-arrow to the right to do that.
It doesn't show you the current download status or tell you when it's done.
The default filename for YouTube videos is apparently just "YouTube.webm" or whatever extension, so if you want to give it a decent name you have to type it out or copy it from the YouTube listing.
It has a ton of options for formats to convert the video to, including at least one audio format, but somehow it doesn't have any good, modern audio formats like m4a, ogg, opus or wma.
It requires you to download and install an external application for it to work, so you don't get the safety of just running an approved and contained FireFox addon, who knows what malware, spyware or adware is in it.
Just about the only good thing about this addon that it's actually able to download videos.
Actually, I take that back.. it only downloads successfully some of the time. Usually on YouTube I get an error similar to this:

Failed aggregating "Paul Anka - Let It Snow - YouTube"

D:\mp3\d:\mp3\christmas\youtube\Paul Anka - Let It Snow - YouTube-IDGEaS6vs8Y.mkv: Invalid argument

I haven't figured out a way around that. I'm lowering my rating from 3 to 2.

Works great for some vids & suddenly stop working, doesn't grab full streaming video anymore

Most of the time doesn't work, even with the external assist app.

Bild hinkt hinterher! Schade.....

useless

Totally unclear when a file will "convert" and have a watermark on it, even if I choose "download" instead of "download and convert". I never want to download something if a watermark is gonna show up, and I'm never gonna pay for something that an open source project does better. Just don't let me convert something if I didn't pay for that feature!!! Companion application was annoying to install as well. If youtube-dl came out with a GUI version for MacOS I'd never use this extension again. On my windows partition it isn't installed.

it needs a companion app and it doesn't install easy.

While I was happy to try it out I found out that you have to install a converter and that converter writes a large QR code over the right corner of the video which in turn made it impossible to follow the video on how to use a program that I was downloading. Total waste compared to other video converters I have used.

It sometimes works for me on most websites (except youtube) .
Very unreliable, specially for slow connections. Suddenly crashes or claims download is finished (but it's not).
You need to check every video manually if it has fully, not corrupted downloaded. Re-starting downloads of such vids is a PITA, sometimes it just don't.
Regarding the "companion app": I can see in the ressource monitor that this app does not send data, it only receives. Means: I didn't catch it spying.

I'm using this add-on for well over a decade, but it hasn't been working on most websites for well over a year now. I never paid much attention to it and assumed it'll be fixed sooner or later. But I guess not. I'm removing this add-on now with a heavy heart. Time to find an alternative.

Hat bislang immer gut funktioniert, aber bei YouTube nun überhaupt nicht mehr. Das Bedienfeld ist mir auch zu unübersichtlich. Schade - habe es nach der Bewertung deinistalliert.

a pagamento forse funziona

Version 7.3.9 does not work with some web pages, while version 6.3.3 works fine with these same pages. I think the team had to make agreements with some websites to remove the download option. So for me, back to version 6.3.3, and above all no more upgrades.

How is this popular? Video quality is absolutely useless, while it does at least render a recording, the HD downloads are with rare exceptions far worse than 720p, and playback is jerky and unmanageable with delays in loading and every time you change part video watching. While I yet to find a HD downloaders that really works with changes youtube has made, this also isn't one. Maybe if pay to register the quality improves along with removing the annoying QC watermarks it adds but I won't test that theory.

Constantly pushing to download an external companion.

Doesn't work well for impartus video tool. The audio and video are mostly out of sync

I get the desire for a dev to make a few $ for a good add-on; but this has been marked as FREE until recently, and now it is NOT, yet this is NOT explained in the description, but only apparent AFTER you've used it, when you see a VERY LARGE QR code embedded in the downloaded video, only removable with a 'conversion license' for $30. Bad form.

"Crippleware" - built on free software but requires the use of an external application which will watermark everything until you provide a $30 premium "donation". If it were legitimately open source I would be inclined to donate since when it works it works. The user interface and WinXP Shareware style are a turn off and I'm back to Youtube-Dl

I'm not happy that the newer versions require payment to remove the QR Code. The older version did not do this. Sticking with 6.3.3