TypeError: content is null in chrome://drupalforfirebug/content/objects/connector.js

Using Drupal For Firebug 6.x-1.x-dev
Firebug 1.12.1
DrupalForFirebug 2.2

I bring up the Firebug Panel, and click on the Drupal tab
If I click on the Drupal sub-tabs, General, SQL, Forms.. an error appears in the Error console (not the firebug one, the Firefox one found with Control-Shift-J)

Each time I click a tab it reports
"TypeError: content is null"
"chrome://drupalforfirebug/content/objects/connector.js"

   // get the currently requested content
      var drupalFirebugContent = content.document.getElementById('drupalforfirebug_' + filterCategory).cloneNode(true);
      //hidden = drupalFirebugContent.getElementsByClassName("content");
      // remove the style="hidded" from the .content elements.
      //for(var i = 0; i < hidden.length; i++) {
      //   hidden[i].removeAttribute("style");
      //}

The drupalforfirebug_* content exists - It works in Chrome - I am able to see the DrupalForFirebug icon and access the reports.