I know you start an internet browser with:

import webbrowser

webbrowser.open("http://www.google.com/")

But how do I specify if I wanted to for example, make it use firefox over the system default (internet explorer)?

You can write

webbrowser.get('firefox').open("http://www.google.com/")

Sometimes it's better to open firefox with subprocess like this

import subprocess as sp
child = sp.Popen(['firefox', '-p',  'myprofile', '-no-remote', "http://www.google.com/"])

this allows you to open firefox with a different profile, or to close firefox later from your program (with child.terminate()).