This is one of the shortest articles ever, almost nothing to explain here.
First, install Selenium2 through easy_install
sudo easy_install selenium
Then, simply go to the /Applications directory and create a symbolic link, because selenium looks for the Firefox binary in a specific path:
$ cd /Applications
$ sudo ln -s FirefoxNightly.app Firefox.app
Time to test:
>>> from selenium import webdriver
>>> ff = webdriver.Firefox()
You should now see a new Firefox instance.