SeleniumHQ/selenium — pull request #54 by terryyin (2013-04-17 23:23:28+00:00)
Title: Firefox 20.0.1 on Windows "TypeError: environment can only contain string" problem

On Windows, if "from __future__ import unicode_literals" os.path.join() will always return unicode. But Firefox 20.0.1 refuse to take unicode in its environment.
The fix is simply convert the unicode back to string.