Fun fact, you're browsing the web as fetched for you, and rendered in a way you find acceptable, by your user agent

https://en.wikipedia.org/wiki/Software_agent

Better page specifically for user-agent in a browser context: https://en.wikipedia.org/wiki/User_agent (although has a lot less content)

Besides Firefox, I don't think many of the browsers are user-agents anymore, but more like corporate-agents, as they don't seem to act in the interest of the software user anymore.