I use pytube to download video from sites such as youtube, but i just tried the add on to firefox called download helper. It will download video also but i don't get sound and the video is slow. When i download same video with pytube it is perfect. What could be the problem?

Then you have had far better luck with the new download helper than I have had.

At one time it was a great program and worked like a charm. Since being "Upgraded" to so-called Video Download Helper, it has been nothing but problematic for me. Most of the time it indicates that it is downloading normally, but when you go to the folder it creates for a download, the folder is empty.