Firefox will not start at all. What do I do?

Firefox will not start at all. A few minutes ago, I had the main Firefox window and Chatzilla open. I closed FF and kept CZ open, then tried to reopen a FF window by double-clicking the shortcut on my desktop. It didn't do anything - I got a loading icon for a moment, then it cuts off. I closed CZ and tried again, and had the same thing. This continued. However, I can still access IE, so it's unlikely to be a virus (most viruses would be more likely to attack IE). How can I fix this? == This happened == Not sure how often == Explained above. == == User Agent == Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Win64; x64; Trident/4.0; .NET CLR 2.0.50727; SLCC2; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0)