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I don’t know if this will help or even if it pertains to the same video download addon in firefox, however, for the firefox “Video Downloand Helper” addon, this was posted on its description page:

If you are having problems downloading videos from YouTube, please check this link:

http://bugzilla.downloadhelper.net:8080/show_bug.cgi?id=536

UPDATE July 19th 2013:

For today’s youtube changes, please follow this procedure for downloading videos that currently generate empty files:

- Open url “about:config”
- Create a new parameter called “dwhelper.yt-sig-perm-85” of type String
- Set its value to “[[2,8],[0,1],[9,21],[65,66],[22,65],[84,85],[66,82],[21,22]]”
(without the quotes).
- Reload the youtube page (no need to restart Firefox) and you should be done.

UPDATE July 22nd 2013:

4.9.18 is waiting for approval at Mozilla. This version automatically adapts to YouTube signature changes (responsible for empty downloads) and fixes a few bugs like video variant ordering and removal of audioless or videoless variants.

UPDATE July 24th 2013:

YouTube changed their signature last night.

Version 4.9.19 is waiting for approval at https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/versions/

Note that since 4.9.18, Video DownloadHelper automatically learns the new signatures, but your first downloads just after a signature change may fail. Just reload the YouTube video page, wait for the video to start playing after the ad and retry downloading a few times (generally 1 to 4 attempts are enough to generate the signature).

UPDATE July 26th 2013:

YouTube changed their signature permutation method again last night.

4.9.19 and 4.9.20 has been removed from the Mozilla review pool.

Current working version is 4.9.21a1 (soon to be submitted to Mozilla as 4.9.21): http://www.downloadhelper.net/install-beta.php?version=4.9.21a1

Details here: http://bugzilla.downloadhelper.net:8080/show_bug.cgi?id=536

which is at:

https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/