Firefox extension, which tracks time spent in Facebook and tells it to you INSTANTLY.

background-script.js

timeSpent = 0;
timeStarts = 0

browser.tabs.onActivated.addListener((activeInfo) => {
    browser.tabs.sendMessage(
        activeInfo.tabId,
        {timeSpent: timeSpent}
    ).then(response => {
        timeStarts = (new Date).getTime();
    }).catch(error => {
        if (timeStarts > 0) {
            timeSpent += (new Date).getTime() - timeStarts;
        }
        timeStarts = 0;
    });
});

manifest.json

{
    "manifest_version": 2,
    "name": "Higher power",
    "version": "1.0",

    "description": "Helps you to avoid temptations",

    "icons": {
    },

    "content_scripts": [
      {
        "matches": ["*://*.facebook.com/*"],
        "js": ["power.js"],
        "run_at": "document_end"
      }
    ],
    "background": {
        "scripts": ["background-script.js"]
    }
  }

power.js

var banner = document.getElementById("mybanner");
if (banner == null) {
    banner = document.createElement("div");
}

banner.id = "mybanner";
banner.innerHTML = "Banner Content";
banner.style.position = "fixed";
banner.style.zIndex = 99999999;
banner.style.display = "block";
banner.style.backgroundColor = "#CCC";
banner.style.top = 2;
banner.style.left = 2;
banner.style.border = "2px solid green";
document.body.insertBefore(banner, document.body.childNodes[0]);

timeSpent = 0;
seconds = 0;

function tick() {
    console.log("tick");
    seconds++;
    spent = (Math.floor(timeSpent/1000) + seconds);
    spentMin = Math.floor(spent / 60);
    spentHours = Math.floor(spentMin / 60);
    repr = "" + spent + "s";
    if (spentMin >= 1) {
        repr = "" + spentMin + "min";
    } else if (spentHours >= 1) {
        repr = "" + spentHours + "h " + spentMin + "min";
    }
    banner.innerHTML = "Spent here: " + repr;
    if (spent > 15*60) {
        banner.style.backgroundColor = "yellow";
    } else if (spent > 60*60) {
        banner.style.backgroundColor = "red";
    }
}

intervalId = setInterval(tick, 1000);

browser.runtime.onMessage.addListener(request => {
    clearInterval(intervalId);
    seconds = 0;
    timeSpent = request.timeSpent;
    intervalId = setInterval(tick, 1000);
});