Upon startup, Firefox creates and listens on a new UNIX socket (shared
accross all tabs and windows) on which commands can be thrown in to fill
HTML forms.  The greeting message indicates that the server is ready to
accept commands.  Each command and response are single UTF8-encoded
lines ending with a newline character `\n'.  Each reponse consists of a
code (OK, ERROR, or BYE), together with an optional message.  Each
command line yields to a single a response line, and commands may not be
sent in parallel: clients must wait for the response of a first command
before sending a second one.

A OK response code indicates a successful greeting or client command,
and may be accompanied with a JSON-encoded message data.  An ERROR
response code indicates a server or client protocol error, the reason of
which may be given as an accompanying UTF8 string.  A BYE response code
indicates an imminent end to client connection.

    OK [' ' JSON-encoded-data]
    ERROR ' ' UTF8String
    BYE


The greeting message consists of the the URI (scheme://hostname:port) of
the active tab of the active window.  (":port" is ommited for default
ports, e.g., 443 for https.)  Said URI is then cached for the whole
session, utils the client disconnects or an REFRESH command is issued.
Hence subsequent GETFORMS and FILL commands are always relative to the
greeting or the most recent REFRESH response URI, and not the possibly
new active tab.  The URI is JSON-encoded (and decodes as a string not an
object, which is an extension to RFC4627) hence non-printable and
control characters are excaped properly (but they are not valid in a
hostname anyway).

    S: OK ' ' JSON-encoded-URI


The REFRESH command clears the internal URI and HTML form caches, and
the server replies with the URI of the currently (active) tab of the
(currently) active window.  See the greeting message above for details.

    C: REFRESH
    S: OK ' ' JSON-encoded-URI


The GETFORMS command yields a response message data consisting of the list of
all visible forms, with their method, action URI, and for each field of type
password, text or email, their name, type, value and maxLength (unless -1)
values:

    [
      {
        "method": "POST",
        "action": "https://mail.fripost.org/",
        "fields": [
          {
            "name": "_user",
            "type": "text",
            "value": "guilhem"
          },
          {
            "name": "_pass",
            "type": "password",
            "value": "top secret"
            "maxLength": 32
          }
        ]
      }
    ]

The form list is kept in cache until the client closes the connection or
another GETFORMS command is issued.  This ensure that subsequent FILL commands
are relatve to these forms even if the document has been modified.

    C: GETFORMS
    S: OK JSON-encoded-array


The FILL command must be preceeded by a GETFORM command, and takes an index and
a JSON-encoded array of string as arguments.  The index is that of the form one
wants to fill, and the array must be at most as long as there are of fields in
that form (as found in the most recent GETFORMS response).  The value of
each such field is then updated with that found in the array (unless
null).

    C: FILL index JSON-encoded-array
    S: OK


The QUIT command yields a BYE response.

    R: QUIT
    S: BYE



Example:

    S: OK "https://mail.fripost.org"
    C: GETFORMS
    S: OK [{"method":"POST","action":"https://mail.fripost.org/","fields":[{"name":"_user","type":"text","value":""},{"name":"_pass","type":"password","value":""}]}]
    C: FILL 0 ["guilhem","topsecret"]
    S: OK
    C: GETFORM
    S: ERROR Invalid command: GETFORM
    C: GETFORMS
    S: OK [{"method":"POST","action":"https://mail.fripost.org/","fields":[{"name":"_user","type":"text","value":"guilhem"},{"name":"_pass","type":"password","value":"topsecret"}]}]
    C: QUIT
    S: BYE