You will have to install the add-on for the Firefox, there are DownThemAll, FlashGot and many more add-ons available for the purpose of download acceleration.Visit the webpage https://addons.mozilla.org/en-US/firefox
Click on the Most popular list, or search for above mentioned add-ons or for any of the download accelerator add-on you know, and Click on "+ Add to Firefox" button.
Firefox will download the add-on, Then select the add-on and press Install and you are done.
