The 1609 first folio edition contains the first printing of the mutability cantos.
Ralph Church: "A new and fair Edition of those six Books in Folio, some few years after the Death of Spenser, appeared in 1609 with the Addition of 'Two Cantos of Mutability, and the Beginning of a third.' This Copy is distinguished, in our Notes, by L. 1. and, in the Cantos of Mutability, is our Standard. Here it must be owned that some pains were taken, but unhappily, as I apprehend, directed to a wrong object. For instead of endeavouring to clear and settle the Text, the Managers of that Edition only clogged and perplexed it, in my opinion, by the Addition of many thousand stops which are not in the preceding Copies: and presuming, as it should seem, that the last Impression of the three first Bookes was most correct, they printed from that second without so much as once consulting the first. What number of Copies were then printed off, we know not; but the Demand for the Poem was certainly great; whereas a second Edition in Folio (but not so fair a one) appeared, the first three Books in 1611, and the Remainder either in 1612, or 1613" Faerie Queene (1758) 1:v.
Henry John Todd: "There is no preface to this edition. I have sometimes thought, that Gabriel Harvey might be the editor. But, probably, he would have furnished us with further information in regard to the 'never-before-imprinted' Cantos, if he had undertaken such an office" Works of Spenser (1805) 1:clxxvii.
Jewel Wurtsbaugh: "Spenser died in 1599. Ten years later Mathew Lownes published the first folio of the Faerie Queene (1609), now augmented by 'Two Cantos of Mutabilite' together with a third 'unperfite.' From whom or how Lownes secured this unpublished portion of the poem, and by what authority the cantos were numbered, whether there was a note or notes upon the manuscript to that effect, are unknown" Two Centuries of Spenserian Scholarship (1936) 2.