Can you please tell me what "first" is in the following sentence.

It was up to another American, Robert Fulton, to develop the first widely successful steamboat.

I think "first" is an adjective modifying steamboat. Thanks, Kris

You are correct

thank you so much!

Just a rule of thumb: Adverbs answer the questions -- when, where, how, how much, how many, why, or negative/positive.