Ele usa um argumento de linha de comando, não um arquivo. Eu acredito que isso seja verdade no Firefox compilado para macs também.
$ firefox --help
Usage: firefox [ options ... ] [URL]
where options include:
X11 options
--display=DISPLAY X display to use
--sync Make X calls synchronous
--g-fatal-warnings Make all warnings fatal
Firefox options
-h or -help Print this message.
-v or -version Print Firefox version.
-P <profile> Start with <profile>.
-migration Start with migration wizard.
-ProfileManager Start with ProfileManager.
-no-remote Open new instance, not a new window in running instance.
-UILocale <locale> Start with <locale> resources as UI Locale.
-safe-mode Disables extensions and themes for this session.
-jsconsole Open the Error console.
-browser Open a browser window.
-new-window <url> Open <url> in a new window.
-new-tab <url> Open <url> in a new tab.
-preferences Open Preferences dialog.
-search <term> Search <term> with your default search engine.
-private Enable private browsing mode.
-private-toggle Toggle private browsing mode.
-setDefaultBrowser Set this app as the default browser.