People are taking that and another reference to mean that Nathan Fillion is playing Cable in the sequel. Or it could be a ‘Two Guys and a Girl’ injoke. The trailer is what you call NSFW.
The Firefly bit is around the two minute mark.
This post was originally published as "Firefly posters in Deadpool 2 teaser trailer spark speculation." by What Would Malcolm Reynolds Do? at Whedonesque