gecko-dev/remote/doc/Usage.md

Usage

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.

As the remote agent is not compiled into Firefox by default, you will first have to get a build that has it. You can read more about self-servicing such a build in the developer documentation, under Building.

To check if your Firefox build has support for the new CDP-based remote debugger, you can check if its help message includes this:

% ./firefox -h
…
  --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.
…

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

[<host>][:<port>]

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:

localhost:9222
127.0.0.1:9999
[::1]:4567
:0

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 (127.0.0.1) and third ([::1]) examples will, respecitvely, force the HTTP to listen on IPv4 or IPv6.

The fourth example will use the default hostname, which is 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 atomatically allocate an arbitrary free port.

Allocating an atomic port can be useful if you want to be cautious race conditions. The atomically allocated port will be somewhere in the ephemeral port range, which varies depending on your system and system configuration.

As the atomic port is bound arbitrarily, there is intrisincly no way to detect upfront what it is. However, Firefox will write the port it bound to, both to stdout and to the remote.httpd.port preference in the current profile’s user.js file.