Hi, can you give us a bit more information than just that, like the OS your using, the version of Firefox, exactly what happen, do you get an error. That might give us a bit more information for us to help :) Haven you installed an addon recently? That might be a reason.

John

Right-click the Firefox icon on the taskbar and select Open New Window. If it opens, close it by Menu > File > Exit. After a short time, try again.