I have the DownloadHelper in Mozilla and Internet Explorer. With that extension I can download *.flv videos from a web (not youtube). But I discovered that when I play the video that I downloaded, the quality is not the same as the original video. I lose quality. The only way to download the videos that I want is via DownloadHelper. I tried with the convert option to different formats and I cant get the same quality as in the original web page. Why? I dont know. Maybe is a codec thing? or I need to do something special? please give me your opinion. Thank you.
Most video sites support multiple streams which vary in quality. There is not a single video file to download, they actually have more than one and you get served the best quality you can receive.
So, if you are not getting the same quality with DownloadHelper then either DownloadHelper is not communicating properly with the website, or the website knows you are using DownloadHelper and giving you the poorest quality video.
Your issue may be that you are playing the video back in a larger window than what you play it in the web player. If you play a video and blow up the size of the windows it plays in it will look fuzzy and broken up. Match the windows size and you may find that the quality is the same.
And what Phil said above is also true and may be the cause of this if it's not a simple player windows size issue.