diff --git a/layout/html/document/src/forms.css b/layout/html/document/src/forms.css
index 07857e750d95..a39fa02f846d 100644
--- a/layout/html/document/src/forms.css
+++ b/layout/html/document/src/forms.css
@@ -161,7 +161,7 @@ select > option {
   padding: 0 5px 0 3px;
 }
 
-option[-moz-option-selected] {
+option[_moz-option-selected] {
   color: HighlightText ! important;
   background-color: Highlight ! important;
 }
diff --git a/layout/style/forms.css b/layout/style/forms.css
index 07857e750d95..a39fa02f846d 100644
--- a/layout/style/forms.css
+++ b/layout/style/forms.css
@@ -161,7 +161,7 @@ select > option {
   padding: 0 5px 0 3px;
 }
 
-option[-moz-option-selected] {
+option[_moz-option-selected] {
   color: HighlightText ! important;
   background-color: Highlight ! important;
 }