The beginnings of a Thunderbird extension to convert an email to a RememberTheMilk task.

rtm-thunderbird.js

// License: GPLv3+
// Author:  2019 Daniel Barrows
// Status:  pre-alpha
// Canonical URL: https://gist.github.com/daniel-barrows/c4345cfc22f3e8e6032d130ab11cbdf4

// require jquery
// Tested with jquery 1.12.4 and jquery-ui 1.12.1 at https://jqueryui.com/resources/demos/dialog/default.html
// Require Thunderlink

// Feature request:
// When any email is sent, tag: tasked-for-later should be removed, or perhaps
// prompt for removal

// These are options that should be set in a preferences dialog
rtm_thunderbird = {
  prefs: {
    default_task_entry:     "Reply to %SENDER% re: %SUBJECT%",
    hidden_task_additions:  "#email %URL%",
    rtm_task_email:         'username+abc123@rmilk.com',
    archive_default:        true,
  }
};

// This should be pulled from the email metadata
email_meta = {
  "%SENDER%": 'Joe Brown',
  "%SUBJECT%": 'Meeting request',
  '%URL%':     'thunderlink://messageid=81f59-1795-5c6df840@example.com',
};

// This is the actual script.

// Based on https://dxr.mozilla.org/thunderbird/source/mail/base/content/mailContextMenus.js#780
// This successfully opens a compose window with appropriate headers, but I wish
// it could send the message as well.
function compose_email_now(headers){
  var fields = Components.classes["@mozilla.org/messengercompose/composefields;1"].createInstance(Components.interfaces.nsIMsgCompFields);
  var params = Components.classes["@mozilla.org/messengercompose/composeparams;1"].createInstance(Components.interfaces.nsIMsgComposeParams);
  for(var k in headers) fields[k]=headers[k];
  params.type = Components.interfaces.nsIMsgCompType.New;
  params.format = Components.interfaces.nsIMsgCompFormat.Default;
  //params.identity = accountManager.getFirstIdentityForServer(GetLoadedMsgFolder().server);
  params.identity = accountManager.getFirstIdentityForServer(this.GetSelectedMsgFolders()[0].server);
  params.composeFields = fields;
  var msgComposeService = Components.classes["@mozilla.org/messengercompose;1"].getService(Components.interfaces.nsIMsgComposeService);
  return msgComposeService.OpenComposeWindowWithParams(null, params);
  // See https://dxr.mozilla.org/thunderbird/source/mail/base/content/mailCommands.js#180
  //ComposeMessage(Components.interfaces.nsIMsgCompType.New,
  //               Components.interfaces.nsIMsgCompFormat.Default,
  //               GetDefaultAccountRootFolder(),
  //               params); // params is ignored here.
  //let mheader = Components.classes['@mozilla.org/messenger;1'].getService(Components.interfaces.nsIMessenger).msgHdrFromURI(?????)
}

// This should return true if and when the email is stored in the outbox.
// https://www.rememberthemilk.com/services/email/
rtm_thunderbird.add_task = function(task_str){
  headers = {
    to:      rtm_thunderbird.prefs.rtm_task_email,
    subject: task_str,
  }
  compose_email_now(headers);
};

// https://stackoverflow.com/a/7975025/5389585
rtm_thunderbird.replace_with_email_meta = function(str, email_meta){
  return str.replace(/%\w+%/g, function(all) {
     return email_meta[all] || all;
  });
};

rtm_thunderbird.prompt_task_for_email = function(email_meta){

  task_entry           = rtm_thunderbird.replace_with_email_meta(rtm_thunderbird.prefs.default_task_entry, email_meta);
  hidden_task_addition = rtm_thunderbird.replace_with_email_meta(rtm_thunderbird.prefs.hidden_task_additions, email_meta);
  archive_checked      = rtm_thunderbird.prefs.archive_default ? 'checked="checked"' : ''

  dialog = jQuery('<div/>', {
      id: 'rtm-taskmaker-dialog',
      title: 'Create RememberTheMilk task from email',
  });
  dialog.append(`<label>Enter task: <input type="text" id="rtm-task-entry" required minlength="1" value="${task_entry}" size="96%" /></label><br/>
                 <label><input type="checkbox" id="rtm-email-archive" ${archive_checked}" /> Archive associated email?</label>`);
  dialog.appendTo('body');
  dialog.dialog({
    'width': '60em',
    buttons: [
      {
        text: "Add task",
        icon: "ui-icon-plus",
        click: function() {
          task_entry = $('#rtm-task-entry').val() + ' ' + hidden_task_addition;
          // Don't wait for mail to be sent, but do wait for mail to be stored.
          if(rtm_thunderbird.add_task(task_entry)){
            // email.addTag() and email.archive() are pseudo-code.
            email.addTag('tasked-for-later');
            if($('#rtm-email-archive').prop('checked')){email.archive()};
          }
          alert(task_entry);
          $( this ).dialog( "close" );
        }
      },
      {
        text: "Cancel",
        icon: "ui-icon-cancel",
        click: function() {
          $( this ).dialog( "close" );
        }
      },
    ]
  });

  return null;

};

//rtm_thunderbird.prompt_task_for_email(email_meta);