aboutaddons.js

function startup(data) {
  Services.obs.addObserver(ExtensionManager, "EM-loaded", false);
}
function shutdown(reason) {
  Services.obs.removeObserver(ExtensionManager, "EM-loaded", false);
}

let ExtensionManager = {
  observe: function(aSubject, aTopic, aData) {
    // aSubject will be the window that about:addons is opened in
  }
}