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Headless mode is a very useful way to run Firefox. Just as it might sound, Firefox is run as normal, minus any visible UI components visible. Though not so useful for surfing the web, it comes into its own with automated testing. This article provides all you need to know about running headless Firefox.

Using headless mode

This section provides usage instructions for headless mode.

Basic usage

You can run Firefox in headless mode from the command line, by including the -headless flag. For example:

You need to run your headless Firefox with a new profile, otherwise you'll run into an error because you can't have two instances open with the same profile.

To create a new profile, go to the about:profiles page in Firefox and use the Create a New Profile button.

Once created, you can specify running firefox with the new profile usig the -P flag followed by the name of the profile:

Taking screenshots

Since Firefox 57, the --screenshot flag allows you to take screenshots of websites. The basic usage is as follows:

This creates a full-height screenshot of https://developer.mozilla.com/en-US/ called screenshot.png, in the active directory, with a viewport width of 800px.

You can omit -headless when using --screenshot, as it is implied:

To override the default values, mentioned above, you can use the following flags/features:

• --screenshot name url — Set a custom name for the screenshot by including it between the --screenshot flag and the URL you want to capture. You can specify other web-compatible image formats such as .jpg, .bmp, etc.
• --window-size=x — Set a custom viewport width when taking the screenshot (full height is maintained). Note that the single argument version of this doesn't work.
• --window-size=x,y — Set a custom viewport width and height to capture.

For example, the following command creates a screenshot of https://developer.mozilla.com, in the active directory called test.jpg, with a viewport width of 800px, and a height of 1000px:

Note: There is a bug whereby taking a screenshot can sometimes fail if the specified URL is redirected (see Headless doesn't work when redirect is involved). Make sure you specify the final URL that you want to screenshot.

Automated testing with headless mode

The most useful way to use headless Firefox, is to run automated tests. You can make your testing process much more efficient.

Selenium in Node.js

Here we'll create a Selenium test, using Node.js and the selenium-webdriver package. For this guide, we'll assume that you already have basic familiarity with Selenium, Webdriver, and Node, and you already have a testing environment created. If not, work through our Setting up Selenium in Node guide, and return when you have.

First, confirm you've installed Node and the selenium-webdriver on your system. Then create a new file, called selenium-test.js, and follow the steps below to populate it with test code.

Note: Alternatively, you could clone our headless-examples repo. This also includes a package file, so you can just use npm install to install necessary dependencies.

1. Let's add some code. Inside this file, start by importing the main selenium-webdriver module, and the firefox submodule:

2. Next, we create a new options object, and add the -headless argument to tell the driver instance we create below to run in headless mode:

Now let's create a new driver instance for Firefox, using setFirefoxOptions() to include our options object:

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1. Alternatively, you can use options to set the binary and the headless arguments:

2. Finally, add the following code, which performs a simple test on the Google search homepage:

3. Finally, run your test with following command:

That's it! After a few seconds, you should see the message 'Test passed' returned in the console.

Headless Firefox in Node.js with selenium-webdriver, by Myk Melez, contains additional useful tips and tricks for running Node.js Selenium tests with headless mode.

Selenium in Java

Note: Thanks a lot to nicholasdipiazza for writing these instructions!

This guide assumes you already have Geckodriver on your machine, as explained in Setting up Selenium in Node, and an IDE set up which supports Gradle projects.

1. Download our headlessfirefox-gradle.zip archive (see the source here). Extract it, and import the headlessfirefox folder into your IDE, as a gradle project.

2. Edit the build.gradle file, to set selenium to a later version, if needed. At the time of writing, we used 3.5.3.

3. Edit the webdriver.gecko.driver property, in the HeadlessFirefoxSeleniumExample.java file, to equal the path where you installed geckodriver (see line 15 below).

4. Run the java class, and you should see the HTML content of this page printed in your console/terminal.

Selenium in Python

This guide assumes you already have geckodriver on your machine, as explained in Setting up Selenium in Node.

1. Install the latest version of the Python client for Selenium.

2. Edit the following, to set the executable_path on line 11, to the path where you installed geckodriver:

3. Run the Python script, and you should see the HTML content of this page printed in your console/terminal.

Other testing solutions

• Slimerjs has Firefox support built in on Linux, with Mac and Windows support, coming soon. See Headless SlimerJS with Firefox by Brendan Dahl for more details.
• TestCafe (v.0.18.0 and higher) also supports testing in headless Firefox, by default. See the documentation for the details.

In addition, you can use headless Firefox to run automated tests written in most other popular testing apps, as long as you are able to set environment variables.

Debugging headless Firefox

You need to set a few preferences to allow remote debugging:

The example above shows the capabilities block required for WebdriverIO, other tools would require the same configuration. Then you will need to follow the remote debugging instructions.

Troubleshooting and further help

If you are having trouble getting headless mode to work, then do not worry — we are here to help. This section is designed to be added to as more questions arise, and answers are found.

• On Linux, certain libraries are currently required on your system, even though headless mode doesn't use them, as Firefox links against them. See bug 1372998, for more details and progress towards a fix.

If you want to ask the engineers a question, the best place to go is the #headless channel on Mozilla IRC. If you are pretty sure you've found a bug, file it on Mozilla Bugzilla.

See also

• Using Selenium with Headless Firefox (on Windows) by Andre Perunicic (uses Python)
• Headless Firefox in Node.js with selenium-webdriver by Myk Melez
• Headless SlimerJS with Firefox by Brendan Dahl
• Using Selenium with Headless Firefox on Travis-CI by Josef Rousek