Dev : Tips : Enabling dump
From MozillaZine Knowledge Base
Viewing dump() output
You can use dump() statements instead of alert() to display some debug information for your extension. In order to view the dump() output, go to about:config, add new boolean pref 'browser.dom.window.dump.enabled' and set its value to 'true'.
Then start Firefox up with 'firefox.exe -console' command. You will now be able to see dump() output in the opened console window.
See also: Dev_:_Tips_:_Printing_to_JSConsole