as mentioned by others recently, this doesn't work yet for the recent versions of Firefox 39 and 40 - it doesn't seem to have been updated since 2013 December 29 (Version 18.104.22.168) - so it is approaching 2 years out of date.
It started to have issues up until at least Firefox 36 (but still worked via a work-around (start download, view download queue, then close and reopen Firefox).
At the present it doesn't even allow the download button to be enabled.
working version would have had 4-5 stars
currently only 2 or less since it doesn't work
reply to Vincent Duvernet's comment October 3, 2015
I tried video-downloadhelper ... it also is unable to do Dailymotion
so I am suspecting the site, Dailymotion, might have changed their permissions to block this sort of thing?