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 or html5.

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:77.0) Gecko/20100101 Firefox/78.0

This will create a user agent that identifies as Firefox 78 on Windows 10.