Drupal for Firebug does not work in FF> Works fine in Chrome
There are some issues with the Drupal for Firebug plugin depending on which version of Firefox/Firebug/DrupalForFirebug you are running.  See the following issue on DO Extension breaks with version 14 of Firefox

It works for me on:

Firefox - 18.0.2
Firebug - 1.11.2
Drupal for Firebug Module - 7.x-1.4
Drupal for Firebug Firefox Plugin - 2.2