Selenium install Marionette webdriver
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:
Open a terminal and run this command
export PATH=$PATH:/Users/myproject/geckodriver-0.8.0-OSX
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.
In addition to the other two answers, you probably don't want to change the PATH system wide since you need it only when running the tests. A way to have the right PATH only when you need it is to set it in code:
os.environ["PATH"] += os.pathsep + 'path/to/dir/containing/geckodriver/'A simpler workaround would be to simple move the geckodriver binary to the directory you already have in your path:
mv geckodriver /usr/local/bin