User Agent Spoofing

When a browser or bot requests a webpage, it sends a User-Agent HTTP header containing a string of text that describes the client. So, if you are using Firefox on Windows to browse the web, it is sending this string of text to every website you visit, where it is visible by the server and any embedded third-party scripts like banner advertising:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:142.0) Gecko/20100101 Firefox/142.0