I LOVE this addon!
I have never had a problem downloading any vid from youtube.
Maybe, you can add more sites to download from.
I can't find any reliable downloader for vimeo, for example...
I was in panic, when I opened up my firefox a while ago and almost ALL of my addons have been disabled! :O
I'm grateful to the reviewer who mentioned this and the prompt response of DFVT explaining this issue - I can't lose this addon!! 3> 3> 3>