Getting the currently selected text in a Firefox extension

firefox-extension-get-selected-text.md

Post e10s, this seems to be the most consistent way (should work in both pre and post-e10s builds):

const tabs = require("sdk/tabs");

function getSelectedText() {
    return new Promise(function (resolve, reject) {
        tabs.activeTab.attach({
            contentScript: "self.postMessage(String(getSelection()));",
            onMessage: resolve,
            onError: reject,
        });
    });
}

Then you can use it pretty simply:

getSelectedText().then(text => {
    console.info("SELECTED TEXT =>", text);
});

Hello. I am trying for more than 3 weeks to get the selected text in Firefox. Can you explain me how to use this. Thanks