发现是一波三折，刚开始信步漫游，就又遇上了沟，整理一下吧：
1、WebDriverException: Message: 'geckodriver' executable needs to be in PATH.
下载firefox驱动geckodriver可以放到python安装目录C:Python27，https://github.com/mozilla/geckodriver/releases
2、WebDriverException: Message: Unsupported Marionette protocol version 2, required 3
升级Mozilla Firefox >= v47
3、WebDriverException: Message: Expected browser binary location, but unable to find binary in default location, no 'moz:firefoxOptions.binary' capability provided, and no binary flag set on the command line
把firefox的安装路径加入到环境变量path，使用的firefox 49，selenium3.0.2验证OK。