If we're comparing combos only, Firecracker wins it quite easily for me.
Senzo has never really been all about combos, he focuses more on making a cool cinematic animation with smooth movements and nice visuals. He's definitely one of my favorites just because he's so damn good at what he's doing, but since he never really tries to take too much time on animating impressive combos rather than just making a good animation as a whole, they don't really stand out too much as they do in Firecracker's or Lithium's animations.
With that said, Lithium's combos are pretty good for what they are. However, due to his old-school style, he really hasn't developed very well and his combos are lacking a lot from what I've seen. Let's take the most recent example I could find;
http://i57.tinypic.com/2q01po7.jpgI know this isn't his best work but it represents his style quite well as a whole. It's snappy and very nice to look at, but not really what I would consider an "impressive combo". The stickfigures are quite stiff and look quite unnatural at multiple frames which really takes away from the animation as a whole. The snappiness is way too exaggerated and makes it look like the stickfigures have no weight at all, and it removes all force the hits could've had. It also felt very spastic which bothered me quite a bit.
Not that it's all bad though. It felt very lively with the bounciness and the choreography was nice. I just feel like it doesn't reach the standards that have been put up by the top Pivot animators that we have now.
Firecracker obviously has his flaws too, I think most of them have been pointed out. However, what he has above most others is that he really knows how to add weight and force in his hits. He also really knows his choreography and it looks quite realistic, although his style isn't actually that close to real martial arts. Plus, the combos in Firecracker vs Hbomb were insane. Like, damn.