If someone's deceased spouse was previously married but that marriage ended in their first spouse's death, would their current surviving spouse need to provide a death certificate for their deceased spouse's previous spouse when applying for widow's benefits?

Sorry for the late answer. SSA's survivor application asks how and when any earlier marriages ended, and SSA can ask for proof, such as a certified copy of the first spouse's death certificate. If that's hard to get, SSA accepts other evidence when you explain why the certificate isn't available. Apply now with what you have rather than waiting to gather it, and SSA will tell you what else it needs.