Mozilla firefox is an open-source alternative webbrowser. In plain English that means, it is a free program that allows you to look at the internet (just like internet explorer).
However, it being open-source does not only mean that it is free of charge, it also means that people are legally free to alter the program as they see fit. So there are a lot of "add-ons" that you can install to tweak the program. One of these add-ons is "ad-block plus" which blocks adds on websites. You see the same page as you would see on internet explorer, only the adds isn't displayed and the content of the website is moved up or down accordingly, so that there isn't an "open gap" either.