YouTube may have changed the way such videos are served.

Can you post a link to a page with a video where the extensions aren't working?

This is one of them.

http://www.youtube.com/watch?v=Zzyfcys1aLM

Yes, that doesn't seem to be working with the DLH extension.
What worked in this case is the "Pop out" player.
That allowed to save the video as webm format and I was able to play this file.

Can you post a link to the Pop-Out player?

You can right-click the video player to see this in the right-click context menu.

• http://kb.mozillazine.org/Prevent_websites_from_disabling_new_window_features

There are websites that have managed to fix this problem. See http://www.themp3tube.com

It was a Youtube update that messed things up a few days ago.

Try Youtube Video and Audio Downloader , https://addons.mozilla.org/en-US/firefox/addon/youtube-video-and-audio-dow/

I think we have a winner : D Thanks, Andy.

Unlike mp3tube, that particular add-on lets you download the video in full WebM format : )

Thanks Andy. Works here too

This worked until this morning. VEVO vids will not download anymore.

Try it here :) https://github.com/inbasic/iaextractor/issues/51

Awesome. Thanks, Joe.

I noticed that KeepVid.com is working again too. That was what I was using before this whole mess started.

Hopefully there's no more problems from here on out, but I'm expecting there will be. But as long as there's at least one way to download these videos in full WebM format, I'll be more than happy. Thanks to everyone involved : )

dvdvideosoft.com managed to fix the vevo problem too, you can try their YouTube downloader

Can someone explain the manual way to download new VEVO videos? I was always able to download Youtube videos by inspecting network traffic in Firebug and there was an HTTP GET request to the FLV file. Is the video link hidden in the source code of the HTML tag or is the video hashed in several parts?

Yes YouTube keeps updating their website and some changes in the new web page code is preventing firefox add-ons from working. The author of the add-on needs to release an update to make it work. There are other apps that will do this that are absolutely free. I personally like "Download YouTube to MP3" it is updated on the regular and seems to always work. Download YouTube to MP3 I'm sure you already have but just-in-case you haven't make sure you download the latest version for firefox of the extension or add-on that is not working ;)

There is a online video downloader can download VEVO videos. And this video downloader has two version. One of them is for Windows, other for Mac, including Windows 8 and Mavericks.