In Unix systems (of which OS X is one), options to programs are given at the command line, normally preceeded by "-". So, to run firefox with, for example a private session, open a terminal and enter:
$ firefox -private
If you want to make this the default, every time you run firefox, you can create a BASH alias. Edit the file
.bashrc in your user's home directory and add this line:
`alias firefox="firefox -private"`
Once you save that file, everytime you run firefox you will run it with the desired options.
Finally, to get a list of the command line options available to firefox, run
firefox -h from a terminal. The following is the output on Linux:
$ firefox -h
Usage: firefox [ options ... ] [URL]
where options include:
--display=DISPLAY X display to use
--sync Make X calls synchronous
--g-fatal-warnings Make all warnings fatal
-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 Do not accept or send remote commands; implies -new-instance.
-new-instance 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.