Why is the video i downloaded much poorer quality than the original? How can i keep the good quality?

so i used the "ook video ook" add-on from firefox to download a video from vidiac.com. It was an "flv" file, which i used media-convert.com to convert to an "avi" file. But when i compare the avi file to the original video on vidiac, the original is a much higher quality. is there a way to get the original good quality back? i have done the same thing many other times w/ other videos, and the quality is always the same. i'm not sure why this one is any different