Index: /CKEditor/trunk/CHANGES.html
===================================================================
--- /CKEditor/trunk/CHANGES.html	(revision 7601)
+++ /CKEditor/trunk/CHANGES.html	(revision 7602)
@@ -52,4 +52,5 @@
 		<li><a href="http://dev.ckeditor.com/ticket/9097">#9097</a> : [IE] Fixed small selection flaw when select start from the blank region outside body.</li>
 		<li><a href="http://dev.ckeditor.com/ticket/9129">#9129</a> : Fixed various Del/Backspace keystroke behavior inside of HTML list.</li>
+		<li><a href="http://dev.ckeditor.com/ticket/9117">#9117</a> : [FF] Avoid js error when calling setData() on a hidden editor.</li>
 	</ul>
 	<h3>
Index: /CKEditor/trunk/_source/core/dom/element.js
===================================================================
--- /CKEditor/trunk/_source/core/dom/element.js	(revision 7601)
+++ /CKEditor/trunk/_source/core/dom/element.js	(revision 7602)
@@ -505,5 +505,8 @@
 				function( propertyName )
 				{
-					return this.getWindow().$.getComputedStyle( this.$, '' ).getPropertyValue( propertyName );
+					var style = this.getWindow().$.getComputedStyle( this.$, null );
+
+					// Firefox may return null if we call the above on a hidden iframe. (#9117)
+					return style ? style.getPropertyValue( propertyName ) : '';
 				},