Perversion/misspelling of "First".
When a new YouTube video from a popular uploader gets online and no comment has been left, people generally try to be first in commenting.
Similar to taking a girls virginity, being first to comment on a video seems to be quite thrilling to most.
Since the thrill is very high, concerned people type so quickly that they misspell "First", resulting in "Firts" or "Frist".
The end result is usually in lower case letters.