main.js

const context = {
    port: null,
}

const messages = [
    "ping",
    "pong",
    "foobar",
    "Ф",
];
let index = 0;

function initNativePort() {
    const port = context.port = browser.runtime.connectNative('webextdev');
    port.onMessage.addListener((aResponse) => {
        console.log('Native message', aResponse);
    });
}

browser.browserAction.onClicked.addListener(() => {
    if (!context.port) {
        initNativePort();
    }

    console.log('Sending native message');
    const cMessage = messages[index % messages.length];
    ++index;
    context.port.postMessage(cMessage);
});