function handleUpdated(tabId, changeInfo, tabInfo) { if (changeInfo.url) { var http = "http://www.youtube.com/watch?v="; var https = "https://www.youtube.com/watch?v="; var url = changeInfo.url; if (url.includes(http) || url.includes(https)) { browser.tabs.insertCSS(tabId, { file: "/styles/betterYoutube.css" }); browser.tabs.executeScript(tabId, { file: "/scripts/betterYoutube.js" }); } } } browser.tabs.onUpdated.addListener(handleUpdated);