Ich habe mich entschieden, Mitglied bei der Freiwilligen Feuerwehr zu werden.

Is there a reason Freiwilligen is capitalized here? Normally I would think of this as an adjective, modifying Feuerwehr. I know it can also be a plural noun meaning “Volunteers” but it doesn’t make sense in that way in this sentence. Is this a typo or is there something I’m missing here?