Technical overview

This page is an edited extract from Bill McCloskey's blog post on multiprocess Firefox:

At a very high level, multiprocess Firefox works as follows. The process that starts up when Firefox launches is called the parent process. Initially, this process works similarly to single-process Firefox: it opens a window displaying browser.xul, which contains all the principal UI elements for Firefox. Firefox has a flexible GUI toolkit called XUL that allows GUI elements to be declared and laid out declaratively, similar to web content. Just like web content, the Firefox UI has a window object, which has a document property, and this document contains all the XML elements from browser.xul. All the Firefox menus, toolbars, sidebars, and tabs are XML elements in this document. Each tab element contains a <browser> element to display web content.

The first place where multiprocess Firefox diverges from single-process Firefox is that each <browser> element has a remote="true" attribute. When such a browser element is added to the document, a new content process is started. This process is called a child process. An IPC channel is created that links the parent and child processes. Initially, the child displays about:blank, but the parent can send the child a command to navigate elsewhere.


Somehow, displayed web content needs to get from the child process to the parent and then to the screen. Multiprocess Firefox depends on a new Firefox feature called off main thread compositing (OMTC). In brief, each Firefox window is broken into a series of layers, somewhat similar to layers in Photoshop. Each time Firefox draws, these layers are submitted to a compositor thread that clips and translates the layers and combines them together into a single image that is then drawn.

Layers are structured as a tree. The root layer of the tree is responsible for the entire Firefox window. This layer contains other layers, some of which are responsible for drawing the menus and tabs. One subtree displays all the web content. Web content itself may be broken into multiple layers, but they’re all rooted at a single “content” layer.

In multiprocess Firefox, the content layer subtree is actually a shim. Most of the time, it contains a placeholder node that simply keeps a reference to the IPC link with the child process. The content process retains the actual layer tree for web content. It builds and draws to this layer tree. When it’s done, it sends the structure of its layer tree to the parent process via IPC. Backing buffers are shared with the parent either through shared memory or GPU memory. References to this memory are sent as part of the layer tree. When the parent receives the layer tree, it removes its placeholder content node and replaces it with the actual tree from content. Then it composites and draws as normal. When it’s done, it puts the placeholder back.

The basic architecture of how OMTC works with multiple processes has existed for some time, since it is needed for Firefox OS. However, Matt Woodrow and David Anderson have done a lot of work to get everything working properly on Windows, Mac, and Linux. One of the big challenges for multiprocess Firefox will be getting OMTC enabled on all platforms. Right now, only Macs use it by default.

User input

Events in Firefox work the same way as they do on the web. Namely, there is a DOM tree for the entire window, and events are threaded through this tree in capture and bubbling phases. Imagine that the user clicks on a button on a web page. In single-process Firefox, the root DOM node of the Firefox window gets the first chance to process the event. Then, nodes lower down in the DOM tree get a chance. The event handling proceeds down through to the XUL <browser> element. At this point, nodes in the web page’s DOM tree are given a chance to handle the event, all the way down to the button. The bubble phase follows, running in the opposite order, all the way back up to the root node of the Firefox window.

With multiple processes, event handling works the same way until the <browser> element is hit. At that point, if the event hasn’t been handled yet, it gets sent to the child process by IPC, where handling starts at the root of the content DOM tree. The parent process then waits to run its bubbling phase until the content process has finished handling the event.

Inter-process communication

All IPC happens using the Chromium IPC libraries. Each child process has its own separate IPC link with the parent. Children cannot communicate directly with each other. To prevent deadlocks and to ensure responsiveness, the parent process is not allowed to sit around waiting for messages from the child. However, the child is allowed to block on messages from the parent.

Rather than directly sending packets of data over IPC as one might expect, we use code generation to make the process much nicer. The IPC protocol is defined in IPDL, which sort of stands for “inter-* protocol definition language”. A typical IPDL file is PNecko.ipdl. It defines a set messages and their parameters. Parameters are serialized and included in the message. To send a message M, C++ code just needs to call the method SendM. To receive the message, it implements the method RecvM.

IPDL is used in all the low-level C++ parts of Gecko where IPC is required. In many cases, IPC is just used to forward actions from the child to the parent. This is a common pattern in Gecko:

void AddHistoryEntry(param) {
  if (XRE_GetProcessType() == GeckoProcessType_Content) {
    // If we're in the child, ask the parent to do this for us.

  // Actually add the history entry...

bool RecvAddHistoryEntry(param) {
  // Got a message from the child. Do the work for it.
  return true;

When AddHistoryEntry is called in the child, we detect that we’re inside the child process and send an IPC message to the parent. When the parent receives that message, it calls AddHistoryEntry on its side.

For a more realistic illustration, consider the Places database, which stores visited URLs for populating the awesome bar. Whenever the user visits a URL in the content process, we call this code. Notice the content process check followed by the SendVisitURI call and an immediate return. The message is received here; this code just calls VisitURI in the parent.

The code for IndexedDB, the places database, and HTTP connections all runs in the parent process, and they all use roughly the same proxying mechanism in the child.

Frame scripts

IPDL takes care of passing messages in C++, but much of Firefox is actually written in JavaScript. Instead of using IPDL directly, JavaScript code relies on the message manager to communicate between processes. To use the message manager in JS, you need to get hold of a message manager object. There is a global message manager, message managers for each Firefox window, and message managers for each <browser> element. A message manager can be used to load JS code into the child process and to exchange messages with it.

As a simple example, imagine that we want to be informed every time a load event triggers in web content. We’re not interested in any particular browser or window, so we use the global message manager. The basic process is as follows:

// Get the global message manager.
let mm = Cc[";1"].

// Wait for load event.
mm.addMessageListener("GotLoadEvent", function (msg) {
  dump("Received load event: " + + "\n");

// Load code into the child process to listen for the event.
mm.loadFrameScript("chrome://content/content-script.js", true);

For this to work, we also need to have a file content-script.js:

// Listen for the load event.
addEventListener("load", function (e) {
  // Inform the parent process.
  let docURL = content.document.documentURI;
  sendAsyncMessage("GotLoadEvent", {url: docURL});
}, false);

This file is called a frame script. When the loadFrameScript function call runs, the code for the script is run once for each <browser> element. This includes both remote browsers and regular ones. If we had used a per-window message manager, the code would only be run for the browser elements in that window. Any time a new browser element is added, the script is run automatically (this is the purpose of the true parameter to loadFrameScript). Since the script is run once per browser, it can access the browser’s window object and docshell via the content and docShell globals.

The great thing about frame scripts is that they work in both single-process and multiprocess Firefox. To learn more about the message manager, see the message manager guide.

Cross-process APIs

There are a lot of APIs in Firefox that cross between the parent and child processes. An example is the webNavigation property of XUL <browser> elements. The webNavigation property is an object that provides methods like loadURI, goBack, and goForward. These methods are called in the parent process, but the actions need to happen in the child. First I’ll cover how these methods work in single-process Firefox, and then I’ll describe how we adapted them for multiple processes.

The webNavigation property is defined using the XML Binding Language (XBL). XBL is a declarative language for customizing how XML elements work. Its syntax is a combination of XML and JavaScript. Firefox uses XBL extensively to customize XUL elements like <browser> and <tabbrowser>. The <browser> customizations reside in browser.xml. Here is how browser.webNavigation is defined:

<field name="_webNavigation">null</field>

<property name="webNavigation" readonly="true">
     if (!this._webNavigation)
       this._webNavigation = this.docShell.QueryInterface(Components.interfaces.nsIWebNavigation);
     return this._webNavigation;

This code is invoked whenever JavaScript code in Firefox accesses browser.webNavigation, where browser is some <browser> element. It checks if the result has already been cached in the browser._webNavigation field. If it hasn’t been cached, then it fetches the navigation object based off the browser’s docshell. The docshell is a Firefox-specific object that encapsulates a lot of functionality for loading new pages, navigating back and forth, and saving page history. In multiprocess Firefox, the docshell lives in the child process. Since the webNavigation accessor runs in the parent process, this.docShell above will just return null. As a consequence, this code will fail completely.

One way to fix this problem would be to create a fake docshell in C++ that could be returned. It would operate by sending IPDL messages to the real docshell in the child to get work done. We may eventually take this route in the future. We decided to do the message passing in JavaScript instead, since it’s easier and faster to prototype things there. Rather than change every docshell-using accessor to test if we’re using multiprocess browsing, we decided to create a new XBL binding that applies only to remote <browser> elements. It is called remote-browser.xml, and it extends the existing browser.xml binding.

The remote-browser.xml binding returns a JavaScript shim object whenever anyone uses browser.webNavigation or other similar objects. The shim object is implemented in its own JavaScript module. It uses the message manager to send messages like "WebNavigation:LoadURI" to a content script loaded by remote-browser.xml. The content script performs the actual action.

The shims we provide emulate their real counterparts imperfectly. They offer enough functionality to make Firefox work, but add-ons that use them may find them insufficient. I’ll discuss strategies for making add-ons work in more detail later.

Cross-process object wrappers

The message manager API does not allow the parent process to call sendSyncMessage; that is, the parent is not allowed to wait for a response from the child. It’s detrimental for the parent to wait on the child, since we don’t want the browser UI to be unresponsive because of slow content. However, converting Firefox code to be asynchronous (i.e., to use sendAsyncMessage instead) can sometimes be onerous. As an expedient, we’ve introduced a new primitive that allows code in the parent process to access objects in the child process synchronously.

These objects are called cross-process object wrappers, frequently abbreviated to CPOWs. They’re created using the message manager. Consider this example content script:

addEventListener("load", function (e) {
  let doc = content.document;
  sendAsyncMessage("GotLoadEvent", {}, {document: doc});
}, false);

In this code, we want to be able to send a reference to the document to the parent process. We can’t use the second parameter to sendAsyncMessage to do this: that argument is converted to JSON before it is sent up. The optional third parameter allows us to send object references. Each property of this argument becomes accessible in the parent process as a CPOW. Here’s what the parent code might look like:

let mm = Cc[";1"].

mm.addMessageListener("GotLoadEvent", function (msg) {
  let uri = msg.objects.document.documentURI;
  dump("Received load event: " + uri + "\n");
mm.loadFrameScript("chrome://content/content-script.js", true);

It’s important to realize that we’re send object references. The msg.objects.document object is only a wrapper. The access to its documentURI property sends a synchronous message down to the child asking for the value. The dump statement only happens after a reply has come back from the child.

Because every property access sends a message, CPOWs can be slow to use. There is no caching, so 1,000 accesses to the same property will send 1,000 messages.

Another problem with CPOWs is that they violate some assumptions people might have about message ordering. Consider this code:

mm.addMessageListener("GotLoadEvent", function (msg) {
  mm.sendAsyncMessage("ChangeDocumentURI", {newURI: ""});
  let uri = msg.objects.document.documentURI;
  dump("Received load event: " + uri + "\n");

This code sends a message asking the child to change the current document URI. Then it accesses the current document URI via a CPOW. You might expect the value of uri to come back as "". But it might not. In order to avoid deadlocks, CPOW messages can bypass normal messages and be processed first. It’s possible that the request for the documentURI property will be processed before the "ChangeDocumentURI" message, in which case uri will have some other value.

For this reason, it’s best not to mix CPOWs with normal message manager messages. It’s also a bad idea to use CPOWs for anything security-related, since you may not get results that are consistent with surrounding code that might use the message manager.

Despite these problems, we’ve found CPOWs to be useful for converting certain parts of Firefox to be multiprocess-compatible. It’s best to use them in cases where users are less likely to notice poor responsiveness. As an example, we use CPOWs to implement the context menu that pops up when users right-click on content elements. Whether this code is asynchronous or synchronous, the menu cannot be displayed until content has responded with data about the element that has been clicked. The user is unlikely to notice if, for example, tab animations don’t run while waiting for the menu to pop up. Their only concern is for the menu to come up as quickly as possible, which is entirely gated on the response time of the content process. For this reason, we chose to use CPOWs, since they’re easier than converting the code to be asynchronous.

It’s possible that CPOWs will be phased out in the future. Asynchronous messaging using the message manager gives a user experience that is at least as good as, and often strictly better than, CPOWs. We strongly recommend that people use the message manager over CPOWs when possible. Nevertheless, CPOWs are sometimes useful.