Browser Console

The Browser Console is like the Web Console, but applied to the whole browser rather than a single content tab.

So it logs the same sorts of information as the Web Console - network requests, JavaScript, CSS, and security errors and warnings, and messages explicitly logged by JavaScript code. However, rather than logging this information for a single content tab, it logs information for all content tabs, for add-ons, and for the browser's own code.

Similarly, you can execute JavaScript expressions using the Browser Console. But while the Web Console executes code in the page window scope, the Browser Console executes them in the scope of the browser's chrome window. This means you can interact with all the browser's tabs using the gBrowser global, and even with the XUL used to specify the browser's user interface.

To open the Browser Console, select "Browser Console" from the Web Developer submenu in the Firefox Menu (or Tools menu if you display the menu bar or are on Mac OS X).

From Firefox 27 onwards, you can also start the Browser Console by launching Firefox from the command line and passing the -jsconsole argument:

/Applications/FirefoxAurora.app/Contents/MacOS/firefox-bin -jsconsole

The Browser Console looks like this:

You can see that the Browser Console looks and behaves very much like the Web Console:

• most of the window is occupied by a pane that display messages
• at the top, a toolbar enables you to filter the messages that appear
• at the bottom, a command line interpreter enables you to evaluate JavaScript expressions.

Browser Console logging

The Browser console logs the same sorts of messages as the Web Console:

• HTTP requests
• Warnings and errors (including JavaScript, CSS, security warnings and errors, and messages explicitly logged by JavaScript code using the console API)
• Input/output messages: commands send to the browser via the command line, and the result of executing them.

However, it displays such messages from:

• web content hosted by all browser tabs
• the browser's own code
• add-ons.

Messages from add-ons

The Browser Console displays messages logged by all Firefox add-ons.

Console.jsm

To use the console API from a traditional or bootstrapped add-on, get it from the Console module.

One exported symbol from Console.jsm is "console". Below is an example of how to acess it, which adds a message to the Browser Console.

Components.utils.import("resource://gre/modules/devtools/Console.jsm");
console.log("Hello from Firefox code"); //output messages to the console

Learn more:

• console API reference
• Console.jsm source code in the Mozilla Cross-Reference

HUDService

There is also the HUDService which allows access to the Browse Console. The module is available at Mozilla Cross-Reference. We see we can not only access the Browser Console but also Web Console.

Here is an example on how to clear the contents of the Browser console:

Components.utils.import("resource://gre/modules/devtools/Loader.jsm");
var HUDService = devtools.require("devtools/webconsole/hudservice");

var hud = HUDService.getBrowserConsole();
hud.jsterm.clearOutput(true);

If you would like to access the content document of the Browser Console this can be done with the HUDService. This example here makes it so that when you mouse over the "Clear" button it will clear the Browser Console:

Components.utils.import("resource://gre/modules/devtools/Loader.jsm");
var HUDService = devtools.require("devtools/webconsole/hudservice");

var hud = HUDService.getBrowserConsole();

var clearBtn = hud.chromeWindow.document.querySelector('.webconsole-clear-console-button');
clearBtn.addEventListener('mouseover', function() {
  hud.jsterm.clearOutput(true);
}, false);

Bonus Features Available

For Add-on SDK add-ons, the console API is available automatically. Here's an example add-on that just logs an error when the user clicks a widget:

widget = require("sdk/widget").Widget({
  id: "an-error-happened",
  label: "Error!",
  width: 40,
  content: "Error!",
  onClick: logError
});

function logError() {
  console.error("something went wrong!");
}

If you build this as an XPI file, then open the Browser Console, then open the XPI file in Firefox and install it, you'll see a widget labeled "Error!" in the Add-on bar:

Click the icon. You'll see output like this in the Browser Console:

For Add-on SDK-based add-ons only, the message is prefixed with the name of the add-on ("log-error"), making it easy to find all messages from this add-on using the "Filter output" search box. By default, only error messages are logged to the console, although you can change this in the browser's preferences.

Browser Console command line

Like the Web Console, the command line interpreter enables you to evaluate JavaScript expressions in real time:Also like the Web Console's command line interpreter, this command line supports autocomplete, history, and various keyboard shortcuts and helper commands. If the result of a command is an object, you can click on the object to see its details.

But while the Web Console executes code in the scope of the content window it's attached to, the browser console executes code in the scope of the chrome window of the browser. You can confirm this by evaluating window:

This means you can control the browser: opening, closing tabs and windows and changing the content that they host, and modify the browser's UI by creating, changing and removing XUL elements.

Controlling the browser

The command line interpreter gets access to the tabbrowser object, through the gBrowser global, and that enables you to control the browser through the command line. Try running this code in the Browser Console's command line (remember that to send multiple lines to the Browser Console, use Shift+Enter):

var newTabBrowser = gBrowser.getBrowserForTab(gBrowser.selectedTab);
newTabBrowser.addEventListener("load", function() {
  newTabBrowser.contentDocument.body.innerHTML = "<h1>this page has been eaten</h1>";
}, true);
newTabBrowser.contentDocument.location.href = "https://mozilla.org/";

It adds a listener to the currently selected tab's load event that will eat the new page, then loads a new page.

Modifying the browser UI

Since the global window object is the browser's chrome window, you can also modify the browser's user interface. On Windows, the following code will add a new item to the browser's main menu:

var parent = window.document.getElementById("appmenuPrimaryPane");
var makeTheTea = gBrowser.ownerDocument.defaultView.document.createElementNS("https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "menuitem");
makeTheTea.setAttribute("label", "A nice cup of tea?");
parent.appendChild(makeTheTea);

On OS X, this similar code will add a new item to the "Tools" menu:

var parent = window.document.getElementById("menu_ToolsPopup");
var makeTheTea = gBrowser.ownerDocument.defaultView.document.createElementNS("https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "menuitem");
makeTheTea.setAttribute("label", "A nice cup of tea?");
parent.appendChild(makeTheTea);