When using the CDP-based Remote Agent 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.

Since Firefox 86 the Remote Agent ships in all Firefox releases by default.

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>] 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-debugging-port
DevTools listening on ws://localhost:9222/devtools/browser/7b4e84a4-597f-4839-ac6d-c9e86d16fb83

--remote-debugging-port takes an optional port as input:


You can use this to instruct the Remote Agent to bind to a particular port on your system. port is optional, which means firefox --remote-debugging-port will bind the HTTPD to the default localhost:9222.

If port has been specified the default port will be overridden:

% firefox --remote-debugging-port 9989
DevTools listening on ws://localhost:9989/devtools/browser/b49481af-8ad3-9b4d-b1bf-bb0cdb9a0620

When you ask the Remote Agent to listen on port 0, the system will atomically allocate an arbitrary free port:

% firefox --remote-debugging-port 0
DevTools listening on ws://localhost:59982/devtools/browser/a12b22a9-1b8b-954a-b81f-bd31552d3f1c

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.