I had the same problem. Everything is installed as indicated in the step-by-step installation guide.
I used Symfony 2.2. First, you should know that when using the file there is a Symfony plugin, which can be found at: http://www.symfony-project.org/plugins/sfFirePHPPlugin
look at readme to find out how to install it.
Secondly, I also had a problem with the Monolog FirePHP handler in my config, where he listened to FirePHP Add-On in Firefox, thereby preventing the information from showing. I used a configuration in which all Info level errors were sent to FirePHP in the config_dev file. After removing from configurations, previous FirePHP events began to appear in the FirePHP add-on on Firefox.
Hope this helps you a couple guys :)
Psykehoe
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