there are several reasons why firefox dont use the full stream.
just remember your living in a community, and the people around use the same wires you do, that can greatly reduce the download speed of your connection, it could just be a complete co-incidence that your download speed is cut in half or you could be on the wrong site.
i always use firefox for everything, i stay away from IE altogether, viruses, spam, mealware, trojans etc.
ive had

of bad times with IE.
the internet is like real life traffic, grid locks, trailbacks, creepers etc. so it can slow you download speeds down.