What flashgot is looking for as a download manager is apparently different than what firefox has.
I downloaded, installed it. Got same msg as you. Right clicked on flashgot, flashgot options, more options.
Click on the download manager scroll bar,
its looking for things like
Getright, Leechget, internet download accelerator, downloader 4x, etc.
In short, a 3rd party download manager.
You may also find this page helpfull.
http://www.flashgot.net/features#dmsBTW I have always stayed away from so called download "accelerators" and managers because I've heard they can seriously screw up your net connect for flying AH. I could be wrong, it could just be a myth.
Good luck!