16:05:57 l0nwlf-MBP:/Applications/Firefox.app/Contents/MacOS$ ./firefox-bin 'http://www.google.com'
says firefox is already running as ff is open.
After we quit ff,
16:09:05 l0nwlf-MBP:/Applications/Firefox.app/Contents/MacOS$ ./firefox-bin -remote 'http://www.google.com'
Nothing happens.
16:09:14 l0nwlf-MBP:/Applications/Firefox.app/Contents/MacOS$ ./firefox-bin  'http://www.google.com'
Opened firefox with url(google homepage) as mentioned.

Also,
>>> import webbrowser
>>> url = 'http://www.google.com'
>>> c = webbrowser.get('safari')
>>> c.open(url)
True
Opens two instances of safari, one with home-page and another with url mentioned. Initially safari was not running and firefox is my default browser. Incase safari is running it opens only one instance with url(google homepage) opened.