Bug #237029 --> make sure plugins are disabled by default

mail/app/profile/thunderbird.js

@@ -44,6 +44,7 @@ pref("general.useragent.vendorSub",
 // Overrides of the seamonkey suite mailnews.js prefs
 ///////////////////////////////////////////////////////////////// 
 pref("mail.showFolderPaneColumns", false); // setting to true will allow total/unread/size columns
+pref("mailnews.message_display.allow.plugins", false); // disable plugins by default
 
 // hidden pref for changing how we present attachments in the message pane
 pref("mailnews.attachments.display.largeView", false);