@edsu yes, it connects to an headless firefox via the Marionette protocol
https://github.com/browsh-org/browsh/blob/master/interfacer/src/browsh/firefox.go#L144-L153

and installs into it an extension
https://github.com/browsh-org/browsh/blob/master/interfacer/src/browsh/firefox.go#L186

@hugh