I have this issue with firefox version 47 https://github.com/seleniumhq/selenium/issues/2110
So, i have tried to add Marionette web driver to fix it: https://develop
I ran into this issue and can confirm that firefox_capabilities['binary'] should point to the Firefox binary, not to GeckoDriver. The Python example in the Mozilla WebDriver documentation has been clarified on this topic.
firefox_capabilities['binary']