I am a firefox lover because I use Firefox more than any other browser, but downloading speed normal in firefox is really slow, as if you download the same set of file through a download manager will download the same set of file with in lesser amount of time with much greater transfer rate.
But don’t worry as you can simply achieve the same download speed as you can with any download manager like Internet Download Manager with a small firefox add-on called FireDownlaod.
FireDownlaod lets you supercharge your downloads with some accelerating technology working at the back ground.
Note: Some of the users also reported it blocks the download from some sites so may need to disable it when it does.
But as I have tested at my end it surely increases the download transfer rate some times so my suggestion would be, you can try at least if you want to see the boost up.
Requirements: Requires Firefox 3.0 or greater.