So I've installed FirebugForDrupal (updated)
It seems the extension is not compatible with Firefox 14. The issue report has been opened on July 19, 2012, and it still is open.

The last comments from one of the co-maintainers of the module say:


  unfourtunatly no, currently the new (from the ground up) plugin registers itself with firebug and firefox; it displays the drupal tabs and subtabs. But it doesn't gather the data from the drupalforfirebug drupal module yet.
  
  Once it is ready I will package it as an xpi and allow it to be downloaded from bitbucket while mozilla reviews the update. I should be done sometime this week.





  So now the code on bitbucket is functional. I wouldn't call it testable -- still hanging around pre-alpha.
  
  Before I we call it testable I need to do two things.
  
  
  Further abstract the connection between the Drupal Module and the Firefox Module. Right now the presentation layer calls the connector that returns the content directly. However, once I abstract that connection to an API call layer then changes to the API will be less problematic for Firefox plugin to deal with.
  Remove the dependency on jQuery for the plugin. Currently the plugin loads jQuery to deal with a couple of UI niceties.&lt;-- this is what is blocking the release.
  
  
  By all means though, If you know how to install from code a firefox plugin then please do so. It works just like the old one for the time being.