geckodriver changed the desired capability from `firefox_profile` (https://github.com/pytest-dev/pytest-splinter/blob/8e4cefdf98caff9004b0a574b70866fe2bbbcb1f/pytest_splinter/plugin.py#L307). Instead, per https://github.com/mozilla/geckodriver#firefox-capabilities, `moz:firefoxOptions`/`profile` should be used. I've verified that specfying `moz:firefoxOptions`/`profile` works as intended.