Firefox opens numerous windows upon launch

Hi to all!
I was using Internet Explorer but got intrigued , because I saw my officemates using Firefox as their web browser. I downloaded Firefox 4.0.1. The download, installation and first day of usage went well. The next day, when I launched Firefox, it opened numerous windows. One time, it even reached 20+ blank web pages.
In the meantime, due to this situation, I did not use Firefox and returned to IE8. I am still willing to give Firefox a try, but how do I deal with this problem? I hope somebody will give me insights on the matter at hand.
Thanks!
