"Firebrand" hasn't really had negative connotations for me, although I guess I would see it as an applicable term for any very charismatic, outspoken figure, with either a helpful or harmful stance. I would put Van Jones more under the catagory of "loose cannon". Beck, of course, being possibly the biggest "loose cannon" in the public eye in the U.S. at the moment. (However, I tend to think of a "loose cannon" as an impending threat of possible damage; Beck has gone well past that.)
Strom Thurman should never have been in any influential public position; the fact that he was doesn't mean that it's a good idea to elect or appoint an irresponsible extremist of a different stripe.