Automation: Marionette, WebDriver BiDi, CDP

Firefox supports several automation surfaces, each implementing a different protocol:

Protocol	Implementation	Source
W3C WebDriver (HTTP)	geckodriver ↔ Marionette	testing/geckodriver/ + testing/marionette/
Marionette (legacy)	In-process server	testing/marionette/
WebDriver BiDi	New W3C bidirectional protocol over WebSocket	remote/webdriver-bidi/
CDP (Chrome DevTools Protocol)	Compatibility surface for Chrome-targeting tools	remote/cdp/

The remote/*-protocol implementations live alongside DevTools and share infrastructure.

remote/

remote/
├── server/            # WebSocket server framework
├── shared/            # Common modules (windowtracker, processtracker, ...)
├── webdriver-bidi/    # W3C WebDriver BiDi
├── cdp/               # Chrome DevTools Protocol
├── marionette/        # Browser-side Marionette (Firefox-internal RDP)
├── components/        # Top-level XPCOM components
├── doc/
└── test/

Marionette

Marionette is Mozilla's original browser automation protocol. It was W3C-standardized in modified form as the WebDriver classic protocol. geckodriver translates W3C WebDriver requests into Marionette commands. The protocol is JSON line-delimited over TCP (no HTTP framing).

Marionette lives in two halves:

• The server (in-process, in the browser) — testing/marionette/.
• The out-of-process client (geckodriver) — testing/geckodriver/.

Enable with --marionette on the command line.

WebDriver BiDi

WebDriver BiDi ("Bidirectional WebDriver") is a newer W3C standard that uses WebSockets. It supports event subscriptions, multiple sessions, log streaming, and integrates with browsers without requiring a HTTP-only translation layer.

remote/webdriver-bidi/ implements:

• BrowsingContext module (navigation, tree, captureScreenshot).
• Network module (interception, response/redirect events).
• Script module (evaluate, callFunction, preload scripts).
• Session module (capabilities, status).

geckodriver itself can also speak BiDi to the same server.

CDP compatibility

remote/cdp/ reimplements a subset of Chrome's DevTools Protocol so tools targeting Chrome (Puppeteer, Cypress) can drive Firefox. Coverage is partial; new tools are encouraged to use BiDi instead.

Enabling

• --marionette — enable Marionette server.
• --remote-debugging-port=PORT — CDP / BiDi server.
• Prefs:
  • marionette.enabled
  • remote.active-protocols (e.g. bidi,cdp)

Related

• Geckodriver app
• DevTools