diff --git a/mail/components/addrbook/content/addressbook.xul b/mail/components/addrbook/content/addressbook.xul
index 216af42f2564..be121a0d1e41 100644
--- a/mail/components/addrbook/content/addressbook.xul
+++ b/mail/components/addrbook/content/addressbook.xul
@@ -33,6 +33,7 @@
 
 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+    xmlns:html="http://www.w3.org/1999/xhtml"
     id="addressbookWindow"
     height="450"
     width="750"
diff --git a/mail/components/addrbook/skin/addressbook.css b/mail/components/addrbook/skin/addressbook.css
index f58aca4ca50c..8c4fbbf998e0 100644
--- a/mail/components/addrbook/skin/addressbook.css
+++ b/mail/components/addrbook/skin/addressbook.css
@@ -28,6 +28,7 @@
 @import url("chrome://messenger/skin/");
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
+@namespace html url("http://www.w3.org/1999/xhtml");
 
 /* ::::: tools menu icon ::::: */