Added a preference to control unsigned JAR files for SmartUpdate

modules/libpref/src/init/all.js

@@ -232,6 +232,7 @@ pref("autoupdate.background_download_enabled",  false);
 pref("autoupdate.background_download_directory",  "");
 pref("autoupdate.background_download_byte_range",  3000);
 pref("autoupdate.background_download_interval",  10000);
+pref("autoupdate.unsigned_jar_support",  false);
 
 pref("imap.io.mac.logging", false);