test_1965.py

import unittest
from selenium import webdriver
from selenium.webdriver.firefox.firefox_binary import FirefoxBinary
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities
from selenium.webdriver.firefox.webdriver import WebDriver as FirefoxDriver
from selenium.webdriver.firefox.options import Options

NIGHTLY = '/path/to/nightly/firefox'
CAPS = DesiredCapabilities.FIREFOX
CAPS["marionette"] = True
CAPS["binary"] = NIGHTLY


class CustomBinaryPathTest(unittest.TestCase):

    def test_geckodriver_binary(self):
        driver = webdriver.Firefox(capabilities=CAPS)
        self.assertEqual(NIGHTLY, driver.binary)
        driver.quit()

    def test_options_binary_loc_obj(self):
        options = Options()
        options.binary_location = FirefoxBinary(NIGHTLY)
        driver = FirefoxDriver(firefox_options=options)
        self.assertEqual(NIGHTLY, driver.binary)
        driver.quit()

    def test_options_binary_loc_obj_gecko(self):
        options = Options()
        options.binary_location = FirefoxBinary(NIGHTLY)
        driver = FirefoxDriver(firefox_options=options, capabilities=CAPS)
        self.assertEqual(NIGHTLY, driver.binary)
        driver.quit()

    def test_options_binary_loc_str(self):
        options = Options()
        options.binary_location = NIGHTLY
        driver = FirefoxDriver(firefox_options=options)
        self.assertEqual(NIGHTLY, driver.binary)
        driver.quit()

    def test_options_binary_loc_str_gecko(self):
        options = Options()
        options.binary_location = NIGHTLY
        driver = FirefoxDriver(firefox_options=options, capabilities=CAPS)
        self.assertEqual(NIGHTLY, driver.binary)
        driver.quit()

    def test_firefox_binary_str(self):
        driver = webdriver.Firefox(firefox_binary=NIGHTLY)
        self.assertEqual(NIGHTLY, driver.binary)
        driver.quit()

    def test_firefox_binary_str_gecko(self):
        driver = webdriver.Firefox(firefox_binary=NIGHTLY, capabilities=CAPS)
        self.assertEqual(NIGHTLY, driver.binary)
        driver.quit()

    def test_firefox_binary_obj(self):
        ffbin = FirefoxBinary(NIGHTLY)
        driver = webdriver.Firefox(firefox_binary=ffbin)
        self.assertEqual(ffbin, driver.binary)
        driver.quit()

    def test_firefox_binary_obj_gecko(self):
        ffbin = FirefoxBinary(NIGHTLY)
        driver = webdriver.Firefox(firefox_binary=ffbin, capabilities=CAPS)
        self.assertEqual(ffbin, driver.binary)
        driver.quit()

if __name__ == "__main__":
    pass

Tests to check the status of SeleniumHQ/selenium#1965.
All the tests pass with Selenium>=2.53.3, including 3.0.0b2 (with geckodriver 0.10.0).
NIGHTLY should point to nightly Firefox binary, which should be downloaded separately.