Complex sentence: The first and third Firebrands, Gilbert and Dennison, both of whom were enemies of Iron Man, are much better known than the second Firebrand, Broxtel, who made only a handful of appearances.
Split into simpler sentences: The first and third Firebrands, Gilbert and Dennison, are much better known than the second Firebrand; both were enemies of Iron Man. The second Firebrand, Broxtel, made only a handful of appearances.