Well, as always we can perform this task easily using add-ons and the name of this awesome add-on is 'FireDownload' which boosts your downloading Speed in Browser.
Supercharge your web downloads! By using state of the art acceleration technology, this extension will speed up your downloads up to 10 times.
As far as I am concerned, it increased my normal Firefox Download Speed. And if increased, then you should too give it a try. Install it by following the below link:-
Open your Firefox browser and navigate to the link given above. You will see FireDownload there. Below it click on "Install for Windows" and Install the add-on and Enjoy!!
Download this plugin from here.