I'm working on a browser extension, and I need specific code to run when the new window is loaded. The below code works perfectly in Chrome/Edge:

window.addEventListener('load', (event) => {
  handle_data();
});

This event is never triggered in the Firefox version. Alternatively, I tried:

window.onload = handle_data;

But this is called before the DOM of the page is loaded. Any idea why this is an issue in Firefox?

EDIT: The script is executed using the following:

function open_tab() {
  chrome.tabs.create({
    url: "https://www.test.ca/apd/myad",
    active: false
  }, function (tab) {
    chrome.tabs.executeScript(tab.id, { file: "manage_content_script.js" });
  });
}

And is called on the click of the extension, using the following code in background.js:

if (msg.content == "clicked") {
      chrome.tabs.executeScript(null, { file: "dom_script.js" });
      open_tab();
    }