add width:100% to vbox for webkit / FF consistency

closes  #3049

IPython/frontend/html/notebook/static/less/flexible-box-model.less

@@ -34,6 +34,8 b''
 	display: box;
 	box-orient: vertical;
 	box-align: stretch;
+	/* width must be 100% to force FF to behave like webkit */
+	width: 100%;
 }
 
 .vbox > * {