Firefox 3.0+ supports Components.utils.reportError(). This function send a debugging message to the Error Console.

Components.utils.reportError("a debugging message"); yields following message to the Error Console:

If you don't hope debugging message displayed as a error, nsIConsoleService#logStringMessage() is better. 

Components.classes["@mozilla.org/consoleservice;1"]
 .getService(Components.interfaces.nsIConsoleService)
 .logStringMessage("a debugging message");

yields following message to the Error Console:

This way is too long to write!

Use of dump() is shortest, but you need to run firefox with terminal (command prompt).

Conclusion

reportError() logStringMessage() dump()
Output Error Console as a error message Error Console as a plain message Terminal (Command prompt)
Length of code Not long Long Very short

See also