diff --git a/IPython/frontend/html/notebook/static/css/fbm.css b/IPython/frontend/html/notebook/static/css/fbm.css
index 0e21d19..a40de6f 100644
--- a/IPython/frontend/html/notebook/static/css/fbm.css
+++ b/IPython/frontend/html/notebook/static/css/fbm.css
@@ -34,6 +34,8 @@
 	display: box;
 	box-orient: vertical;
 	box-align: stretch;
+	/* width must be 100% to force FF to behave like webkit */
+	width: 100%;
 }
  
 .vbox > * {