When using the CDP-based remote debugger in Firefox, there are three different programs/components running simultaneously:

  • the client, being the out-of-process script or library (such as Puppeteer) or web inspector frontend you use to control and retrieve information out of Firefox;

  • the agent that the client connects to which is an HTTPD living inside Firefox, facilitating communication between clients and targets;

  • and the target, which is the web document being debugging.

The remote agent ships in Firefox Nightly only.

To check if your Firefox binary has the remote agent enabled, you can look in its help message for this:

% ./firefox -h
  --remote-debugging-port <port>
  --remote-debugger [<host>][:<port>] Start the Firefox remote agent, which is
                     a low-level debugging interface based on the CDP protocol.
                     Defaults to listen on localhost:9222.

When used, the remote agent will start an HTTP server and print a message on stderr with the location of the main target’s WebSocket listener:

% firefox --remote-debugger
DevTools listening on ws://localhost:9222/devtools/browser/7b4e84a4-597f-4839-ac6d-c9e86d16fb83

As you will tell from the flag description, --remote-debugger takes an optional address spec as input:


You can use this to instruct the remote agent to bind to a particular interface and port on your system. Either host and port are optional, which means ./firefox --remote-debugger will bind the HTTPD to the default localhost:9222.

Other examples of address specs include:


The use of localhost in the first example above will, depending on whether the system supports IPv6, bind to both IP layers and accept incoming connections from either IPv4 or IPv6. The second ( and third ([::1]) examples will, respectively, force the HTTP to listen on IPv4 or IPv6.

The fourth example will use the default hostname, localhost, to listen on all available IP layers, but override the default port with the special purpose port 0. When you ask the remote agent to listen on port 0, the system will atomically allocate an arbitrary free port.

Allocating an atomic port can be useful if you want to avoid race conditions. The atomically allocated port will be somewhere in the ephemeral port range, which varies depending on your system and system configuration, but is always guaranteed to be free thus eliminating the risk of binding to a port that is already in use.