The obnoxious practice of responding to a blog post/article/video with a comment saying "first." Generally this comment will have no substance; it is about nothing other than itself. When the first spot is stolen by a non-firster a firster will often resort to "second!" or "third!"
Blogger17: Long, thoughtful post about something important
annoyedbyidiots42: damn firsters, firsting all over the place
Fucking your first cousin.
There weren't any bitches left at the party, so I decided firsting was my only option.