diff --git a/mailnews/addrbook/resources/content/abCardOverlay.xul b/mailnews/addrbook/resources/content/abCardOverlay.xul
index b066d76a0e37..7a1e8fae70e6 100644
--- a/mailnews/addrbook/resources/content/abCardOverlay.xul
+++ b/mailnews/addrbook/resources/content/abCardOverlay.xul
@@ -147,6 +147,7 @@
             </vbox>
             <menulist id="PreferMailFormatPopup" disableforreadonly="true">
               <menupopup>
+                <!-- 0,1,2 come from nsIAbPreferMailFormat in nsIAbCard.idl -->
                 <menuitem value="0" label="&Unknown.label;"/>
                 <menuitem value="1" label="&PlainText.label;"/>
                 <menuitem value="2" label="&HTML.label;"/>