mtab/extend/browserExt/src/background.js

chrome.runtime.onInstalled.addListener((detail) => {
    if (detail.reason === chrome.runtime.OnInstalledReason.INSTALL) {
        if (navigator.userAgent.includes("Firefox")) {
            chrome.tabs.create({
                url: "dist/index.html",
            });
        } else {
            chrome.tabs.create({
                url: "chrome://newtab",
            });
        }
    }
});