I really like this extension along its companion downloader (Free and Open Source)! First what I like, followed by what I fear could be a "bug" (thus -1 star, but it's related to my own personal case).

1- You can enable/disable the URL download link catching process, simply by clicking its toggle button on the toolbar (off= browser's downloader, on=use the companion software JDownloader2).

2- You can rightclick its icon in the toolbar and make it display a list of all available links on a page, and then select those you want to download.

3- For Youtube or others video/music clips, if you have the external app already started, simply copy its URL, and JDownloader's app will catch it - no need to paste it. It will automatically put the different files (srt, video and audio) into a package of links, to the highest resolution, from wich you can then select to download video and audio seperatly and/or to lower resolutions (i.e. you can select to encode audio in AAC or OGG at different bit rates, videos in mp4, Webm or MKV at different resolutions/fps, etc.). Or the assembled clip video+audio by selecting the package itself (rather than selecting the included links to files): it will automatically build an mp4 along the subtitles srt file if present.

4- If you copy a URL link of a clip that comes from a Youtube Playlist, it asks you whether you want to download only the clip you're looking at or the whole list, which is awesome.

NOTES about Youtube AUDIO:
- For portability, OGG at 128Kbps is almost (imperceptibly) equivalent in quality to AAC at 192Kbps, although it takes up much less space (similar to an MP3 compression in terms of file size, but of a way better quality). If you have room, then go for AAC/m4a at 192Kbps
- OGG at 128Kbps is of a much better audio quality than AAC/m4a at 128Kbps, in which case it should be your choice: if the original is OGG 128Kbps, 152Kbps etc., keep that original; or if you want to save space by compressing an original AAC/m4a 192Kbps to 128Kbps, choose OGG.
- If you really need smaller audio files with bitrates lower than 128Kbps (i.e. 94Kbps or 64Kbps), than choose AAC/m4a. When considering the same bitrates, OGG offers a better quality than AAC at above 94Kbps (128Kbps and above).
- If the original is AAC/m4a at 128Kbps, do not convert it to OGG 128 Kbps: you will not gain any space; converting a lossy compression to another lossy codec will result in a slight loss of quality, even if the bitrate is the same (since the compression algorithms are different).

If you already have the stand alone JDownloader2 application running (an excellent customizable and rich Free Open Source program you can download from their Web site), it works fine. Else, either you always open the external application before hand, either you do what Dr.Ne0Gen3tic explains in a previous post (second under this one). Note that you DO NOT need to create an account on JDownloader's Web site, even if you must do what's mentioned in Dr.Ne0Gen3tic's post or on the help page that would get automatically diplayed in your browser.

What I really like is that you can control what to save from a Youtube clip. When not auto-saving, either you click on the container package and click the start button - JDownloader will automatically assemble the files and convert into an mp4 along saving any .srt subtitle file, as it would do in auto-save mode. Either you download only the AUDIO part: right -click on the .m4a or.OGG file, select "Youtube.com" from the contextual menu, then "Audio Variant for 1 link", which will offer to choose between Oggvorbis (.ogg) formats or AAC variants at different bit rates. Then select "start download" from the contextual menu of that .m4a file only. Note that .ogg audio format is of a much better quality/compression than mp3 format, and is Open Source!


LITTLE BUG?
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I always open jdownloader before hand: the purpose of this extension/addon is to use an external downloader so that you can manage, interrup/recover your downloads rather than use your browser's internal downloader, thus it's not a big deal for me.

But still, I'd like the external downloader to start automatically whenever I click a dowload link in the browser and it's not opened yet, HOWEVER: personally I already have the latest NODEJS installed on my PC, the complete package from Microsoft (I do Webapps/site developments and use VS Code from time to time). There's no problem on that side of my environment, but oddly, the Browser extension doesn't seem to find nor address it in order to open JDownloader's app (note that I've installed JDownloader app on another drive than C, on E:\Program Files\JDownloader).

Thus, I'm afraid that installing the additional package (localised nodejs?), which modifies Windows registry entries related to nodeJS from what I've read (please correct me if I'm wrong), will have the potential to collide with my current NodeJS environment. I don't have time to verify what's actually modified, hence I won't install it and simply start JDownloader's app beforehand.