about:debugging

If you are looking for information on the about:debugging page as it existed before Firefox 68, please go to: about:debugging (before Firefox 68) for more information. This page now covers the about:debugging page as it is in Firefox 68 and later.

The about:debugging page provides a single place from which you can attach the Firefox Developer Tools to a number of debugging targets. At the moment it supports three main sorts of targets: restartless add-ons, tabs, and workers.

Opening the about:debugging page

There are two ways to open about:debugging:

  • Type "about:debugging" in the Firefox URL bar.
  • In the Tools > Web Developer menu, click "Remote Debugging".

When about:debugging opens, on the left-hand side, you'll see a sidebar with two options and information about your remote debugging setup:

Setup
Use the Setup tab to configure the connection to your remote device.
This Firefox
Provides information about temporary extensions you have loaded for debugging, extensions that are installed in Firefox, the tabs that you currently have open, and service workers running on Firefox.

Setup tab

Connecting to a remote device

Firefox supports debugging over USB with Android devices.

Before you connect:

  1. enable the Developer Menu on your Android device.
  2. Enable USB debugging in the Android Developer Menu.
  3. Enable USB Debugging in Firefox on the Android device.
  4. Connect the Android device to your computer using a USB cable.

If your device doesn't appear in the lefthand side of the page, try clicking the "Refresh devices" button. If it still doesn't appear and you have followed the above steps, take a look at this troublshooting page.

To start a debugging session, first open the page that you wish to debug and then click "Connect" to open a connection to your device. If the connection was successful, you can now click on the name of your device to switch to a tab with information about your device.

The information on this page is the same as the information on the "This Firefox" tab, but instead of displaying information for your computer, it displays the information for the remote device with the addition of a Tabs section with an entry for each of the tabs open on the remote device.

Note: If the version of Firefox on your remote device is more than one major version older than the version running on your computer, you will a message like the following:

In the image above, there are three tabs open: Network or cache Recipe, Nightly Home, and About Nightly. To debug the contents of one of these tabs, click the "Inspect" button next to its title. When you do, the Developer Tools will open in a new tab.

Above the usual list of tools, you will see information about the device you are connected to, including the fact that you are connected via USB, to Mozilla Firefox Nightly, on a Moto G(5), the title of the page that you are debugging, and the address of the page.

This Firefox

The "This Firefox" tab combines the features of Extensions, Tabs, and Workers into a single tab that has been divided into the following sections:

Temporary Extensions
Displays a list of the extensions that you have loaded using the Load Temporary Add-on... button.
Extensions
This section lists information about the extensions that you have installed on your system.
Service Workers, Shared Workers, and Other Workers
There are three sections on this page that deal with Service Workers, Shared Workers, and Other Workers.

Whether or not internal extensions appear in the list on this page depends on the setting of the devtools.aboutdebugging.showHiddenAddons preference. If you need to see these extensions, navigate to about:config and make sure that the preference is set to true.

Extensions

Loading a temporary extension

With the "Load Temporary Add-on..." button you can temporarily load a web extension from a directory on disk. Click the button, navigate to the directory containing the add-on and select its manifest file. The temporary extension will be displayed under the "Temporary Extensions" header.

You don't have to package or sign the extension before loading and it will stay installed until you restart Firefox.

The big advantages of this method, compared with installing an add-on from an XPI, are:

  • you don't have to rebuild an XPI and reinstall when you change the add-on's code
  • you can load an add-on without signing it and without needing to disable signing.

Once you have loaded a temporary extension, you will see the following information about it:

Inspect
Loads the extension in the debugger.
Reload
Reloads the temporary extension. This is handy when you have made changes to the extension.
Remove
Unloads the temporary extension.

Other information about the extension is included as well:

Location
The location the extensions source code on your local system
Extension ID
The temporary id assigned to the extension.
Internal UUID
The internal UUID assigned to the extension.
Manifest URL
If you click the link, the loaded manifest for this extension is loaded in a new tab.

Updating a temporary extension

If you install an extension in this way, what happens when you update the extension?

  • If you change files that are loaded on demand, like content scripts or popups, then changes you make are picked up automatically, and you'll see them the next time the content script is loaded or the popup is shown.
  • For other changes, click the "Reload" button next to the "Remove" button. This does what it says:

Installed Extensions

The permanently installed extensions are listed in the next section, Extensions. For each one, you will see something like the following:

Just as it does with temporarily loaded extensions, the link next to Manifest URL will open the loaded manifest in a new tab.

Note: At the moment, it's recommended that you use the Browser Toolbox, not the Add-on Debugger, for debugging WebExtensions. See Debugging WebExtensions for all the details.

The Add-ons section in about:debugging lists all web extensions that are currently installed. Next to each entry is a button labeled Inspect.

Note: This list may include add-ons that came preinstalled with Firefox.

If you click Inspect, the Add-on Debugger will start in a new tab.

See the page on the Add-on Debugger for all the things you can do with this tool.

Workers

The Workers page shows your workers, categorised as follows:

You can connect the developer tools to each worker, and send push notifications to service workers.

Service worker state

The list of service workers shows the state of the service worker in its lifecycle. Three states are possible:

  • "Registering": this covers all states between the service worker's initial registration, and its assuming control of pages. That is, it subsumes the "installing", "activating", and "waiting" states.
  • "Running": the service worker is currently running. It's installed and activated, and is currently handling events.
  • "Stopped": the service worker is installed and activated, but has been terminated after being idle.

This section uses a simple ServiceWorker demo, hosted at https://serviceworke.rs/push-simple/.

Unregistering service workers

The Unregister button allows you to "unregister" the service worker:

Sending push events to service workers

To debug push notifications, you can set a breakpoint in the push event listener. However, you can also debug push notifications locally, without needing the server. Click the "Push" button to send a push event to the service worker.

Service workers not compatible

A warning message will be displayed at the top of the This Firefox tab if service workers are incompatible with the current browser configuration, and therefore cannot be used or debugged.

Service workers can be unavailable if the dom.serviceWorkers.enable preference is set to false in about:config.