The author quotes Deu. 4:24 verbatim with the exception of replacing the pronoun σου (“your”) with ἡμῶν (“our”) (and omitting the last clause). As the Hebrew lacks a copula, כִּי יָהְוֶה אֱלֹהֶיךָ אֵשׁ אֹכְלָ, it seems the author of Hebrew felt no need to include it either. This is, of course, acceptable Greek, as the copula can be omitted in certain constructions.1
###Footnotes
1 Buttmann, p. 136, §129; Winer, p. 731–734, 2.
###References
Buttmann, Alexander. A Grammar of the New Testament Greek. Trans. Thayer, Joseph Henry. Andover: Draper, 1873.
Winer, George Benedikt. A Treatise on the Grammar of New Testament Greek. 3rd ed. Trans. Moulton, William Fiddian. Edinburgh: Clark, 1882.