Why do multiple windows screens open when I open my Mozilla Browser? Several screens are blank.

Sounds like you have some spyware on your system

To make sure everything is running fine run both anti virus and anti spyware apps in normal AND safe modes. (make sure that they are updated first ! ;)) (AVG is good and free AV)
(A couple of good removal tools are Spybot and Adaware)

ALso an on line virus and spyware scanner is Trend Housecall

Just a note; actively running two AV's on one machine can cause problems.
So if you are thinking about it make sure your current one is disabled first.
SAme thing applies to online scanners as well.

Removal of junk files is easy with CCleaner a free app that does exactly what you need.