Mozilla Firefox Problems!

I am having problems on my Dimension desktop with Mozilla firefox.  When i open it says tat It is unavailable.  I am connected to my internet.  Internet Explorer works and displays websites but Mozilla doesn't.  I uninstalled Mozilla and i reinstalled it to normal settings, and still did not work.  My settings are all correct and I don't know what to do.  I like using Mozilla and am NOT planning on going back to using Internet Explorer 7! 

hopefully someone can help me and help me get my mozilla back to normal