You can use the Firefox developer tools on your desktop to debug code running remotely: in a different process on the same device or on a completely different device. To do this you use Firefox to attach the Toolbox to the remote process, and the Toolbox is then launched in its own window. At the moment the following tools support remote debugging:
This guide explains how to get set up to debug code running in Firefox for Android over USB. The other main application for remote debugging is to debug code running in the Firefox OS Simulator, but that's covered separately, in the Firefox OS Simulator documentation.
This guide's split into two parts: the first part, "Prerequisites" covers stuff you only need to do once, while the second part, "Connecting", covers stuff you need to do each time you connect the device.
First, you'll need:
Next, you'll need to get the desktop and the Android device talking to each other using the adb command-line tool.
To check it worked, open up a command shell on the desktop and type:
adb devices
You should see some output like:
List of devices attached 51800F220F01564 device
(The long hex string will be different.)
If you do, then adb has found your device and you've successfully set up ADB.
Next, you need to enable remote debugging on both the Android device and the desktop.
To enable remote debugging on the device, you need to set the devtools.debugger.remote-enabled preference to true.
Go to about:config in Firefox for Android, type "devtools" into the search box and press the Search key. You'll see all the devtools preferences. Find the devtools.debugger.remote-enabled preference, and press "Toggle".
On Firefox for Android 25 and later, there's a menu item to enable remote debugging. Open the menu, select "Settings", then "Developer tools" (on some Android devices you may need to select "More" to see the "Settings" option). Check the "Remote debugging" box:
The browser will display a notification reminding you to set up port forwarding, which we'll do later on.
On the desktop, remote debugging is enabled by a setting in the Toolbox. Open the Toolbox, click the "Settings" button in the toolbar, and check "Enable remote debugging" in the Settings tab:
Restart the browser, and you'll see a new option in the Web Developer menu labeled "Connect...":
Now you can connect the remote debugging tools to the device. First, attach the device to the desktop with a USB cable, if you haven't already.
Go to a command prompt, and type:
adb forward tcp:6000 tcp:6000
(If you've changed the value the Android device uses for a debugging port, you'll need to adjust this accordingly.)
For Firefox OS, type:
adb forward tcp:6000 localfilesystem:/data/local/debugger-socket
You'll need to reissue this command each time you physically attach desktop and device with the USB cable.
Then go to the Web Developer menu on Firefox, and select "Connect...". You'll see a page that looks like this:
Unless you've changed the port numbers, choose 6000 and press the "Connect" button.
Next you'll see a dialog on the Android device asking you to confirm the connection:
Press "OK". The desktop waits for a few seconds to give you time to acknowledge this dialog: if it times out, just press "Connect" in the desktop dialog again.
Next, the desktop shows you a dialog that looks something like this:
This is asking whether you want to debug web content running in a browser tab, or to debug the browser code itself.
Let's choose to attach to the mozilla.org website. The Toolbox will open in its own window, attached to the Firefox for Android tab that's currently hosting mozilla.org:
The Toolbox, and the tools it hosts, work in just the same way as they do when attached to local content.