Just in case anyone comes here in the future, here is the deal. The Lightning add-on (which is the d

<p>Just in case anyone comes here in the future, here is the deal. </p><p>The Lightning add-on (which is the default calendar add-on) apparently changes the Thunderbird ua string, causing a 403 forbidden on some URLs for the feed. </p><p>This can be circumvented by either disabling the Lightning extension or changing the value of calendar.useragent.extra to blank (Preferences -&gt; Advanced -&gt; Config Editor). </p>