downloadhelper extension stopped functioning. I need to uninstall it but the extension website says to contact Firefox for such help. Will reinstalling it delete and re-create or will I end up with a non-functioning old copy and a functioning new one?

I use a year-old Macintosh with OS 10.5.8. I tried to change the video output format of downloadhelper extension from FLV to a format playable by Macs and Windows computers. In trying that, downloadhelper disappeared from the upper margin of the open screen and is no longer accessible. I want to dis-install it and then reinstall it, but there is no procedure at either the downloadhelper website or the FireFox website to do so. I am afraid to reinstall without first removing the extension. How can I remove it? The downloadhelper site specifically says to ask Firefox how to do it and offers no help.