The Firebrand is an interesting expert class, one that would be rather fun to play if you have the gift of the gab because at the core, this character is a rabble-rouser. He knows all the right buttons to press to get people worked up - and if he gets tired of espionage he could always retire to politics!
There are plenty of roles such an individual could fill, and some are suggested here. However, many might be better suited to an NPC unless your campaign is structured carefully to accommodate your Firebrand. From revolutionary to political activist or corporate raider (just a few of the suggested roles) it's quite easy to see how to weave a plot around him, less easy to know just what to do with him if your plot is about something else entirely.
Understandibly, the core attribute is Charisma, with Wisdom and Intelligence following along behind. There are plenty of interesting and useful class abilities to fit the Firebrand up as a figurehead that can sway the hearts and minds of those who hear him.
The game mechanics you'll need to run this character are provided: the usual table plus suggested feats and three new ones - the 'Cameraderie' tree which is used to enhance group combat by letting members of the team support each other better. There are also ideas for building the character, including different sorts of Firebrands depending on their origins - one who started life as a Pointman might have a completely different approach than one who was a Fixer or a Scientist, for example.
It is an interesting concept, used well in adventures that will make use of the Firebrand's particular talents it could prove memorable.