firmes, in this sentence, is not a verb form, even though it is a homonym with one, so it does not belong in this grammar exercise.
But I think here, ‘firmes,’ describes how they maintain their convictions: firmly. I’m thinking it’s an adjective, and not a verb?
Here it’s an adverb:
a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc. (e.g., gently, quite, then, there).
That’s right! Thanks!