User agent spoofing

User agent spoofing is a tool often used to get access to websites that reject Windows XP or the older browsers that exist on Windows XP. Even though you can get past the user agent detection, sites may still not work if they actually use more modern javascript, html5 or fingerprinting techniques.

Chromium based browsers
In Chromium browsers like 360, you can install this extension on your browser: User-Agent Switcher and Manager. And you can change the User Agent to the one you want.

Gecko/Goanna based browsers
Use about:config and create a entry called general.useragent.override and create a string like

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:90.0) Gecko/20100101 Firefox/90.0

This will create a user agent that identifies as Firefox 90 on Windows 10.

YouTube
 Mozilla/5.0 (Googlebot/2.1)   Mozilla/5.0 (bingbot/1.0)  These are both known to enforce the old non-polymer layout. Seems Google only sniffs for the 'bingbot' and 'Googlebot' tags since anything can be placed after, even comments directed at Google regarding the throttled-in-non-Chromium-browsers Polymer layout