Unable to call a function on page load in firefox extension

Question

I was trying to write this Firefox Extension in Javascript wherein I need to call a specific function on page load. I tried adding the following line of code in contentScript.js but it only works in Chrome Extension.

contentScript.js

window.addEventListener("load", myFunction, false);


function myFunction() {
 // Function body goes here
}

This works fine in Chrome Extension but unfortunately in case of Firefox, I need to refresh the web page again in order to see the changes. On the contrary, I want to call this function as soon as the web page has loaded successfully. Any ideas around this would be much appreciated. Thanks in advance!