For those that didn't use it, Xfire was basically a combination of messenger, voice chat, and a server browser for games back in the day.
As far as I know, it was also one of the earliest ways to stream your gameplay for others to watch. I remember trying it out years before Twitch was around.
It was pretty much used the way people use Discord with a group of friends today. It didn't have servers or anything like that, but you could hop on a call with a couple of buds and play games together.
I played a lot of Halo Custom Edition over Xfire back in the day...