Error starting Firefox: __init__() got an unexpected keyword argument 'desired_capabilities'

Tried running a test w/ Firefox on GCE in Salt Lake City and got some errors.

The testing completed but failed.

Error starting Firefox: init() got an unexpected keyword argument ‘desired_capabilities’

It seemed to work if I tested GCE in Frankfurt, Germany or Chromium in some other region, so it might be something specific w/ the Firefox agent in that region/instance.