I try to develop a small webextension for thunderbird and the following code trows a warning

window.controllers/Controllers is deprecated. Do not use it for UA detection.

In my Backround.js I do this to get an event if someone clicks my compose_action button:

browser.composeAction.onClicked.addListener(async (tab) => {
    await ChangeMail(tab);
});

To be honest, I dont know what there is wrong and therefore have no idea about how to make better.

Any hints on what the issue is?