tabs.connect()

tabId

integer. ID of the tab whose content scripts we want to connect to.

connectInfo Optional

An object with the following properties:

name Optional

string. Will be passed into runtime.onConnect event listeners in content scripts belonging to this extension and running in the specified tab.

frameId Optional

integer. Open a port to a specific frame identified by frameId instead of all frames in the tab.

runtime.Port. A port that can be used to communicate with the content scripts running in the specified tab.

In this example a background script listens for a click on a browser action, then connects to the currently active tab, then sends a message using the Port that's returned from connect():

function connectToTab(tabs) {
  if (tabs.length > 0) {
    let examplePort = browser.tabs.connect(
      tabs[0].id,
      {name: "tabs-connect-example"}
    );
    examplePort.postMessage({greeting: "Hi from background script"});
  }
}

function onError(error) {
  console.log(`Error: ${error}`);
}

browser.browserAction.onClicked.addListener(() => {
  let gettingActive = browser.tabs.query({
    currentWindow: true, active: true
  });
  gettingActive.then(connectToTab, onError);
});