How do I reset my screen size in Mozilla Firefox?

Hello, I have Mozilla Firefox 3.5 I believe and I need to reset the screen size so that a website will take up the entire screen as usual. Right now there is this border that is all black(as if the screen is in a picture frame) and I want to restore it. I tried asking the forum for Mozilla and they said to try to reset the setting with the safe mode of Mozilla which I tried to do a few times but nothing changed. This is very frustrating! Does anyone have experience with this browser and how to reset it? I don't know all the fancy lingo..I just need to be able to do this step by step. Thanks much!