v0.20.1

Fixed

• Avoid attempting to kill Firefox process that has stopped.

  With the change to allow Firefox enough time to shut down in
  0.20.0, geckodriver started unconditionally killing the process
  to reap its exit status. This caused geckodriver to inaccurately
  report a successful Firefox shutdown as a failure.

  The regression should not have caused any functional problems, but
  the termination cause and the exit status are now reported correctly.