>>878449467tv is a browser addon that hosts many extra chat emotes that aren't hosted by Twitch, as well as some extra features, like highlighting first time chatters. It basically replaces words with those emotes (e.g. peepoCheer or Howdy (pic related). You won't be able to see these without the 7tv or Frankerfacez addons. Frankerfacez has more and better features than 7tv, so I recommend that one.
About Mouse's twitch emotes, you can use them in any Twitch channel when you're subbed, but if you want to check them out and download them you can do it in this site:
https://twitchemotes.com/channels/175831187 