gecko-dev/testing/marionette/README

Marionette [ ˌmarɪəˈnɛt] is

  * a puppet worked by strings: the bird bobs up and down like
    a marionette;

  * a person who is easily manipulated or controlled: many officers
    dismissed him as the mayor’s marionette;

  * the remote protocol that lets out-of-process programs communicate
    with, instrument, and control Gecko-based browsers.

Marionette provides interfaces for interacting with both the internal
JavaScript runtime and UI elements of Gecko-based browsers, such
as Firefox and Fennec.  It can control both the chrome- and content
documents, giving a high level of control and ability to replicate,
or emulate, user interaction.

Head on to the Marionette documentation to find out more:

	https://firefox-source-docs.mozilla.org/testing/marionette/marionette/