Firefox version 3.618 is slowing down loading pages. How do I downgrade to the version before this? I have windows xp

Ever since my system installed Firefox 3.618, web pages are very slow to load, switch pages, gmail is slower, and when it first comes up, there is a funny black area on the menu bar where I can barely see the links to click for gmail, etc., but the black bar disappears when it switches to a new page. I have windows xp, and want to downgrade to the version before 3.618. Thank you.