Firefox only makes execCommand("paste") available in trusted contexts aka extensions. You can't execCommand("paste") on a regular web page for fairly obvious security reasons.
I guess there's room for an attack of irritation where a user with something meaningful on their clipboard unwittingly triggers having it overwritten by some nonsense. (Most clipboards effectively being a single mutable item without history or stack/queue behaviour).
Speaking of clipboard shenanigans. Could be imagining it, but I seem to remember a while ago there being something certain websites were doing where if you selected and copied text from them, the text that ended up on your clipboard would have a synthesized paragraph about copyright/attribution appended to it.
