TO FIX THE PROBLEM, I went back to version 1.1.6 which also uses the older detection algorithm and that works great for me (five stars great). Other users may want to do the same until the bugs are worked out of the latest release. On the options page for this extension, I set "Allow automatic updates" to OFF since I don't want 1.1.6 to be replaced by that buggy new version. Once the bugs are fixed, I'll set it back to the Default value again.

The older 1.1.6 version was obtained by going to the bottom left side of the Firefox Add-On page for this extension. You see a clickable link to See All Versions.

If you only get the spinning hourglass, check if you're running version 1.1.7, if so, just downgrade to 1.1.6 again

*EDIT

This is still the best app for downloading videos from Youtube!! It did not work for a while but I want to thank the author for his hard work making it work again!! You are the man!!

Now 1.1.8 is off the available versions and i can confirm 1.1.7.2 with Firefox 96.0.2 Linux Mint all is now working hunky doory doo :) Thanks Feller .. now i can once again put a 5* on this great extension. Thank you.

--- January 27, 2022 ---
The problem that the screen freezes when the download panel is displayed is occurring again.
At this time, the only solution to this problem is to revert to version 1.6 or earlier.
The download speed will be greatly reduced, but let's accept it as it can't be helped now.

--- January 21, 2022 ---
The button under the player is hidden again, but it is possible to call the download panel from the button in the toolbar of the browser.
If the download panel doesn't finish loading, lowering the version to 1.1.7.2 may help.
By the way, the download speed issue has also been resolved since 1.1.7.2.

(The above is a translation of Japanese into English using Google Translate.)

[update 1.25.2022]
1.1.7.2 stopped working for me yesterday, the download button had disappeared again a couple days ago, no probs I could use the toolbar shortcut, but since yesterday the DL links won't populate, the entire tab will lock up and firefox will display "this tab is slowing down your browser". 1.1.8 doesn't work for me; rolling back to 1.1.6 technically works, I get the links and can download, but the speeds are basically useless, again back to downloading at like 50KB/s, which takes forever for even a small video.

I REALLY DO DESPISE GOOGLE AND YOUTUBE.

Siempre me ha gustado esta extension pero ultimamente no ha estado muy eficiente. No esta bajando los videos y no se cual es el problema.

One of the BEST and MOST CRUCIAL Firefox addons EVER! Happily using for years!
(Especially now that recent problems have been fixed.)

Update on 2022-01-22:
1.1.8 doesn't work for me so I had to go back to 1.1.7.2 by (1) scrolling down on the addon's main page until "Version History" is seen (2) clicking on "See all versions" (3) finding older version 1.1.7.2 and clicking link below it "Download File"

Original review from 4 years ago:
I have used this so much for archiving youtube-only content. It works fantastically and cleanly. I used to use Flashgot, but this is an adequate replacement (since I mostly need downloads from youtube).

Thank you RayLo!

FireFox 97 (64-bits); ADD-ON 1.2.1; Its fully works now

It works in 1.1.7.2(previous release of this addon) with the latest firefox version.
You can open download panel through toolbar.

me encanta, lo mejor que he tenido en este tipo de app

This is generally an excellent product, but as others may have said, the latest version is for some reason not working (perhaps due to an update in YT's UI), but you can go back to the versions page and download an earlier version that appears to be working normally. I'm having good results with Ver 1.16.

new updated version is not working yet. i am sure it will get fixed. in the mean time, i downloaded the last stable version and it works fine. i will update once i see the bugs are worked out. don't rate the app 1 star if you can't figure out to remove and reinstall with last working version. not fair to the developer of this great extension. thanks mate for this tool.

I have used this for Many Many years and it has been Great.

At the Moment Version 1.1.8 The Hour Glass either freezes or just keeps spinning.

Rolling back to 1.1.7.2 has fixed it, I was getting slow downloads for months with this version, but after quite a few attempts at completely removing and reinstalling, it now seems to be downloading properly again.

These are the hardest working devs on the Internet. I have been using this plugin forever, and it has worked 90% of the time. The other 10% is YouTube's fault for changing things up, but these guys are always on TOP of it, man. For now, going back to 1.1.6 will fix most of your issues, until they get to the new problems. Just go back to the main page, under "More Information" box, under "Version History" click on "See all versions" and find 1.1.6 and click that download link.

VERY GOOD