The great thing about FlashGot is that it correctly detects and uses various download managers installed on the system. For example, it detected the KDE3 download manager on my machine, KGet. However, I think it's better to use the browser built-in downloader.
FlashGot can be configured to download only certain types of files, or fetch only media files, the menu entries are configurable so you can add or remove specific options. Below are several screenshots:
Downloading media files from YouTube
Choosing default download manager
Install FlashGot from the Mozilla Add-ons page