It depend on your net bandwith by the way. For FireZ situation, your streamyx or ADSL is your limitation.
For example, using ADSL 2 Mbps at my home for torrenting, download, surfing etc, maximum theoritical rate i could get is 2048 kbps which is translate to 256 kBps (byte). Upon run speed test, I might get around 230-245 kBps only. But still within 70% of the promised speed.
However, when i download stuff from net....it really depend on the server to allocate how much kBps per thread. Some small server give you approx 30 to 40 kBps for each thread...which is enough to enjoy streaming movie and also...suit 512 kbps ADSL. That is why, when you use download manager, you feel that 3 thread is more than enough. 40 x 3 kBps = 120 kBps which is your line mostly can't give that kind of speed (provided you are normal user use ADSL 1 Mbps).
So, in my case, if i download using my firefox at this particular server, i only get let say 30 kBps. By using download manager (be it flashget, thunder, FDM, IDM, DAP, Orbit), and make it 10 thread (which is theoritically, 30x10 kBps = 300 kBps), i will fully utilize my 2 Mbps line or 256 kBps download speed.
But, for some fast server like nzones...no need for me to use download manager. Direct download from browser alone will fully occupied my 2 Mbps line. For my office line, i need to use download manager (after office hour) to fully utlize office 24 Mbps line since nzone direct download will give approximately 500-600 kBps...which is lower than max speed of the line.
To make it short, it all depend on the ftp server bandwith allocation per thread setting.
Anyway....Flashget is one of the best download manager.