Browser extension development question: accessing websites' functions?

Question

I'm making a firefox extension which displays a stopwatch at the top of specific websites. I also have a button that resets the stop watch to 00:00:00. I want the extension to automatically reset the timer whenever any function in that website is called.

For example, if a website has a button which changes the website's background color, then every time that button is clicked the timer should reset.

How do I access the functions of various websites in my extension? Also do I have to change the manifest.json file (permissions and stuff)? And once I access them how can I make an eventlistener for every single one of them?

This is my manifest.json file

{
  "name": "timer",
  "version": "1.0",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["content-script.js"],
      "css":["style.css"]
    }
  ],
 
  "manifest_version": 2
}

My javascript

var tim = document.createElement('time');
var button = document.createElement("button");
button.innerHTML = "RESET";

tim.innerText = "00:00:00"

document.body.insertAdjacentElement('beforebegin', tim);
document.body.insertAdjacentElement('beforebegin', button);

button.addEventListener ("click", function() {
tim.textContent = "00:00:00";
    seconds = 0; minutes = 0; hours = 0;
});
//when anything in body changes
let Par = document.body,
  options = {
    attributes: true
  },
  observer = new MutationObserver(restart);

  
var h1 = document.getElementsByTagName('time')[0],
    seconds = 0, minutes = 0, hours = 0,
    t;
    
    
    

function add() {
    seconds++;
    if (seconds >= 60) {
        seconds = 0;
        minutes++;
        if (minutes >= 60) {
            minutes = 0;
            hours++;
        }
    }

    tim.textContent = (hours ? (hours > 9 ? hours : "0" + hours) : "00") + ":" + (minutes ? (minutes > 9 ? minutes : "0" + minutes) : "00") + ":" + (seconds > 9 ? seconds : "0" + seconds);

    timer();
}
function timer() {
    t = setTimeout(add, 1000);
}
timer();

//restart method that resets timer
function restart() {
tim.textContent = "00:00:00";
    seconds = 0; minutes = 0; hours = 0;
}

  observer.observe(Par, options);

Thanks!

EDIT: following @wOxxOm advice, I have tried tracking any changes in body using MutationObserver. The way I have done it must be wrong because it's not working properly. To reiterate, what I want is when any changes in the body happens then the timer resets. Any idea on why the way I did it is wrong? I am quite new with javascript and haven't used MutationObserver before so I really have no idea