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
the firefox binary capability you're setting points to the firefox binary, not the marionette driver binary. You need to add /Users/myproject/geckodriver-0.8.0-OSX to your path as follows:
/Users/myproject/geckodriver-0.8.0-OSX
Open a terminal and run this command
export PATH=$PATH:/Users/myproject/geckodriver-0.8.0-OSX