WebDriver is a remote control interface that enables introspection and control of user agents. As such it can help developers to verify that their websites are working and performing well with all major browsers. The protocol is standardized by the W3C and consists of two separate specifications: WebDriver classic (HTTP) and the new WebDriver BiDi (Bi-Directional).

This newsletter gives an overview of the work we’ve done as part of the Firefox 136 release cycle.

Contributions

Firefox is an open source project, and we are always happy to receive external code contributions to our WebDriver implementation. We want to give special thanks to everyone who filed issues, bugs and submitted patches.

In Firefox 136, several contributors managed to land fixes and improvements in our codebase:

• Liam (ldebeasi) improved the browsingContext.contextDestroyed event in WebDriver BiDi.
• Krzysztof Jan Modras (chrmod) refactored an internal module for handling addons/webextensions.

WebDriver code is written in JavaScript, Python, and Rust so any web developer can contribute! Read how to setup the work environment and check the list of mentored issues for Marionette, or the list of mentored JavaScript bugs for WebDriver BiDi. Join our chatroom if you need any help to get started!

General

Bug Fixes

• Firefox now handles WebSocket port conflicts for the RemoteAgent more efficiently. If the port specified via the --remote-debugging-port command-line argument cannot be acquired within 5 seconds, such as when another Firefox process is already using it, Firefox will now shut down instead of hanging.
• Navigations using the HTTP scheme, triggered by the WebDriver:Navigate command in Marionette or browsingContext.navigate in WebDriver BiDi, will no longer be automatically upgraded to HTTPS. These requests will now remain on HTTP, as originally intended.

WebDriver BiDi

Updated: session.subscribe now returns a subscription ID

The return value of the session.subscribe command has been updated to include a session.Subscription which is a unique ID identifying the event subscription.

Example: adding a global subscription for network events:

-> {
  "method": "session.subscribe",
  "params": {
    "events": ["network"]
  },
  "id": 1
}

<- {
  "type": "success",
  "id": 1,
  "result": {
    "subscription": "9a29c77b-5e7b-43f1-bc6f-20b5228bf207"
  }
}

New: subscriptions parameter for session.unsubscribe

The subscription IDs can be used with the new subscriptions parameters of session.unsubscribe to remove specific event subscriptions. Previously, it was necessary to unsubscribe using the same attributes provided to session.subscribe. And if there were several calls to session.unsubscribe performed from different areas of the test, there could be unexpected side-effects.

With the new subscriptions parameter, only the subscriptions matching the provided subscription IDs will be removed. The parameter must be an array of subscription IDs. If any ID doesn’t match a known (and not yet removed) subscription ID, an InvalidArgumentError is raised.

When subscriptions is provided, the events or contexts parameters should not be present, otherwise an InvalidArgumentError will be raised.

Example: adding a second subscription for a specific context:

-> {
  "method": "session.subscribe",
  "params": {
    "events": ["browsingContext.contextCreated"],
    "contexts": ["304a76e3-e177-4355-b598-9f5f02808556"]
  },
  "id": 2
}

<- {
  "type": "success",
  "id": 2,
  "result": {
    "subscription": "e1a54927-dc48-4301-b219-812ceca13496"
  }
}

Now removing both subscriptions at once:

-> {
  "method": "session.unsubscribe",
  "params": {
    "subscriptions": ["9a29c77b-5e7b-43f1-bc6f-20b5228bf207", "e1a54927-dc48-4301-b219-812ceca13496"]
  },
  "id": 3
}

<- { "type": "success", "id": 3, "result": {} }

Using the subscriptions parameter allows to remove global and context-specific subscriptions simultaneously, without unexpected side effects.

Deprecated: contexts parameter for session.unsubscribe

It remains possible to globally unsubscribe by using only the events parameter. However the additional contexts parameter is now deprecated and will be removed in a future version. To remove context-specific subscriptions, please use the subscriptions parameter instead.

New: userContexts parameter for script.addPreloadScript

The script.addPreloadScript command now supports a userContexts parameter, allowing clients to specify in which user contexts (containers) a script should be automatically loaded — including new browsing contexts created within those user contexts.

Note: If userContexts is provided, the contexts parameter must not be included. Using both at the same time will raise an InvalidArgumentError.

Example: adding a preload script for a specific user context:

-> {
  "method": "script.addPreloadScript",
  "params": {
    "arguments": [], 
    "functionDeclaration": "(arg) => { console.log('Context created in test user context'); }",
    "userContexts": ["528545b6-89d6-486e-a85c-517212654c13"]
  },
  "id": 4
}

<- { "type": "success", "id": 4, "result": { "script": "168cff9c-9de0-44e7-af95-0c478ad2596f" } }

Updated: browsingContext.contextDestroyed event returns full tree

Thanks to Liam’s contribution, the browsingContext.contextDestroyed event now returns the full serialized tree of destroyed contexts — including all its child contexts.

Example: closing a tab with two iframes:

{
  "type": "event",
  "method": "browsingContext.contextDestroyed",
  "params": {
    "children": [
      {
        "children": [],
        "context": "30064771075",
        "originalOpener": null,
        "url": "https://example.bidi.com/frame1",
        "userContext": "1c88a947-3297-4733-9cd7-c16a863a602d"
      },
      {
        "children": [],
        "context": "30064771076",
        "originalOpener": null,
        "url": "about:blank",
        "userContext": "1c88a947-3297-4733-9cd7-c16a863a602d"
      }
    ],
    "context": "ab8c33a7-6bc8-45cc-a068-7cdcef518577",
    "originalOpener": null,
    "url": "https://example.bidi.com/",
    "userContext": "1c88a947-3297-4733-9cd7-c16a863a602d",
    "parent": null
  }
}