"First!" is a Youtube copypasta meme in which if one is early to a video they would spam the comments
saying they are "first".
The meaning behind the meme itself is that saying "first" on a post is like calling dibs on the front seat of your Mom's car, its something you feel good about expressing first, in this case it's being the first to comment on a post. It is unknown how long the trend has been going on, but it's considered to be iconic enough to be a classic meme.