Create an extension that lets a web developer modify the User-Agent string on a persistent per-site, per-tab basis. There's already a user-agent-switcher in the FF-addons (https://addons.mozilla.org/en-US/firefox/addon/59) but it doesn't isolate the user agent per site or per tab. Strong features might include a separation of the browser identification and language support, optional persistence across browsing sessions and a UI that by default hides the UA string behind the name/version of the browser it represents. See [http://www.webaim.org/blog/user-agent-string-history/ this] discussion of the UA's history.