about:debugging

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 several different ways to open about:debugging:

• Type "about:debugging" in the Firefox URL bar.
• New in Firefox 47: in the Tools > Web Developer menu, click "Service Workers".
• New in Firefox 47: click the wrench icon (), which is in the main toolbar or under the Hamburger menu (), then select "Service Workers".

When about:debugging opens, on the left-hand side, you'll see a sidebar enabling you to switch between the two main views: one for add-ons and one for workers.

Add-ons

This page enables you to do two things:

• Load an add-on temporarily from disk
• Connect the Add-on Debugger to any restartless add-ons

Connecting the Add-on Debugger

The Add-ons page in about:debugging lists all restartless add-ons that are currently installed (note that this list may include add-ons that came preinstalled with your Firefox). Next to each entry is a button labeled "Debug".

If the "Debug" button is disabled, check the "Enable add-on debugging" box.

If you click "Debug", you'll see a dialog asking you to accept an incoming connection. Click "OK", and the Add-on Debugger will start in a separate window. Note that sometimes the debugger window is hidden by the main Firefox window.

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

Loading a temporary add-on

With the "Load Temporary Add-on" button you can load any sort of restartless add-on temporarily, from a directory on disk. Just click the button, navigate to the directory containing the add-on's file, and select any file in that directory. The temporary add-on will be displayed under the "Temporary Extensions" header.

You don't have to package or sign the add-on. The add-on 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.

Updating a temporary add-on

If you install the add-on in this way, what happens when you update the add-on's files?

Before Firefox 48

• 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.
• If you change files that stay loaded the whole time, like background scripts, then you can pick up changes you've made by disabling and then re-enabling the add-on in the about:addons page.
• If you change files that are only parsed at install time, like the manifest.json file, you'll need to restart Firefox, then load the add-on again.

Firefox 48 onwards

From Firefox 48 onwards:

• as before: 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.
• there's a better way to handle the other cases: click the "Reload" button next to the "Debug" button. This does what it says:
  • reloading any persistent scripts, such as background scripts
  • parsing the manifest.json file again, so changes to permissions, content_scripts, browser_action or any other keys will take effect.

Tabs

In Firefox 49 onwards, a Tabs page is available in about:debugging — this provides a complete list of all the debuggable tabs open in the current Firefox instance.

Each tab entry has a Debug button next to it — when clicked, this will open up a toolbox specific to that tab, allowing you to debug that tab's contents.

Workers

The Workers page shows your workers, categorised as follows:

• All registered Service Workers
• All registered Shared Workers
• Other workers, including Chrome Workers and Dedicated Workers

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

Service worker state

From Firefox 52, the list of service workers shows the state of the service worker in its lifecycle. Three states are distinguished:

• "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.

Debugging workers

For a service worker, if it is already running, you'll see two buttons next to it, labeled "Debug" and "Push". If it's not already running, you'll see one button, labeled "Start": click this to start the service worker.

Clicking "Debug" connects the JavaScript Debugger and Console to this worker. You can set breakpoints, step through code, watch variables, evaluate code, and so on:

Registering workers

At first, you won't see any workers listed under Service Workers or Shared Workers. As soon as a worker is registered, the listing is updated:

Unregistering service workers

Starting in Firefox 48, you'll see a link named "unregister" next to each registered service worker:

Click the link 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. Just click the "Push" button to send a push event to the service worker:

Service workers not compatible

Service workers can be unavailable for several reasons:

• If you are using a Private Browsing window.
• If your History preference is set to "Never Remember History" or "Always use private browsing mode".
• If the dom.serviceWorkers.enable preference is set to false in about:config.