"For who so firm that cannot be seduced?" Where is the verb in this Shakespeare quotation?
It's not as common in modern usage, but especially in poetry, copulae ("is", "are") and subject pronouns ("he", "they") can be omitted when the context makes the meaning clear. The clause could also be written
for who is so firm that they cannot be seduced?
But this doesn't fit into the meter. (Shakespeare liked iambic meters, i.e. lines of alternating unstressed-stressed syllables: "for who so firm that cannot be seduced?")