Hey remember that copyrighted content can not be downloaded. Try reinstalling the addon if this effect does not arise, I recommend you install the Youtube MP3 Podcaster (you can download files with extencion Mp3,3gp,flv,Mp4 and video full HD if the video has the option).

Im running windows 7 x64, and firefox 64 nightly and it will not download ANY videos off of you tube, including MINE that I put there that I made myself. I don't even get the little down arrow next to the download helper 3 ballon icon. Nor do I see that icon on the you tube page, (like I do in firefox 32 version)

I have tried uninstalling and re-installing several times, both download helper and firefox itself. Im running explorer 10 on my system cause explorer 11 had so many bugs in it.

64 bit version on Windows aren't supported and only are generated to check for compilation issues and do not undergo the testing that the 32 bit version gets, so they can get broken at any time.
If the 32 bit Nightly version works then use that version.

Well, I suggest that both programs (Firefox and download helper ad on) both start thinking about a real 64bit program that works because a lot of people are going to the 64 and I bet MS windblows 9 will NOT support the 32 bit programs when it comes out.

I mean whats the sense of having a 64 bit computer if the software works only in 32 bit mode?!!