Can you see logs in the FirePHP console when visiting http://firephp.org/ ? That is a good baseline test to ensure the extension is working properly.
Any other issues are typically due to the server library not sending any data back. Make sure you have the server library properly integrated into your application.
If this is not the issue I would be happy to help get it working for you if you file a ticket at: https://github.com/firephp/firephp-for-browser-devtools/issues