firefox2koreader/firefoxplugin/options.js

const urlField = document.querySelector("#koreaderurl");

browser.storage.local.get("koreaderurl").then((res) => {
    console.log(res);
    urlField.value = res.koreaderurl || "http://127.0.0.1:17144";
});

document.querySelector("button").onclick = async function(e) {
    await browser.storage.local.set({"koreaderurl": urlField.value});
    e.preventDefault();
}