tab_comm_tab.js

var btn_id = "my_addon_btn";

// create a button and attach to DOM:
// but only if it isnt there already...
if(!document.getElementById(btn_id)){
    var btn = document.createElement("button");
    btn.id = btn_id;
    btn.innerHTML = "Click Me";
    // add an event listener that will call the btnClick func
    btn.addEventListener("click", btnClick, false);
    // add to the tab's DOM
    document.body.appendChild(btn);
}

// handler for the button's click event
function btnClick(){
    // The button was click so tell main.js
    postMessage({
        "type": "click",
        "msg": "You click on the button!"
    });
}

// handle messages from main.js to here....
onMessage = function onMessage(data) {
    if(data.type == "click_complete"){
         console.log("you clicked a button in the dom");
         console.log("we told main.js about it");
         console.log("and this is main.js's response:");
         console.log(" - " + data.msg);
    }else{
        console.log("unknown message type: " + data.toSource());
    }
};