Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third Person Objective (Editor) and First Person (Pamela)
The fictional editor who presents Pamela's letters to us comments on Pamela's life and fate from a position outside the situations he describes, so he's definitely a third person (objective) narrator. He is careful to emphasize his own objectivity in the preface, when he says, "an Editor may reasonably be supposed to judge with an Impartiality which is rarely to be met with in an Author towards his own Works" (1.11).
The editor is not really a frame narrator, since we don't really learn anything about him or his life. However, the conceit that Pamela's letters have been collected and vetted by an objective third party is important for at least two reasons: (1) it adds to the illusion that Pamela's letters are real, thereby increasing the story's verisimilitude and (2) it gives Richardson a venue for presenting some the novel's moral messages.
If you missed some of the novel's not-so-subtle instructive messages, the editor is here to help.
Editor aside, the lion's share of the story consists of Pamela's first-person narration, offered via her letters to her parents. The main story opens with just such a letter, with Pamela lamenting "I have great Trouble, and some Comfort, to acquaint you with" (4.1), and on it rolls from there. For 500-ish pages. But who was counting?