Well, I don't have a Mac, but others of the Firebug Working Group like Honza do and test it regularily there. Note, that you can always open Firebug via F12. Of course that's not solving your problem, but please read the following and have a look, if that helps you:
http://blog.getfirebug.com/2011/08/03/firebug-start-button/If not, please create a new Firefox profile and just install Firebug and let us know, if that fixes the problem.
Greetings
Sebastian