diff -Nru a/components/SOGoConnectorAutocompleteSearch.js b/components/SOGoConnectorAutocompleteSearch.js
--- a/components/SOGoConnectorAutocompleteSearch.js	2014-10-01 09:13:36.000000000 +0200
+++ b/components/SOGoConnectorAutocompleteSearch.js	2016-05-02 17:50:44.000000000 +0200
@@ -118,7 +118,7 @@
     // Other numbers currently unused (hence default to zero)
     _parser: MailServices.headerParser,
     _abManager: MailServices.ab,
-    applicableHeaders: Set(["addr_to", "addr_cc", "addr_bcc", "addr_reply"]),
+    applicableHeaders: new Set(["addr_to", "addr_cc", "addr_bcc", "addr_reply"]),
 
     // Private methods