I just installed and used the Firestarter firewall management tool. I can categorically say that Firestarter is fantastic.
But, if you're already using a traffic shaping solution to improve interactivity and latency, such as Wondershaper, don't use Firestarter's Quality of Service filtering at all.
Because Wondershaper beats Firestarter on that department hands down. Sure, Firestarter sets the firewall rules that priorize (in my case, interactive) traffic. But that's just not enough. For cable modem users (where cable modems have deep inbound/outbound traffic queues), the upload rate limiter in Wondershaper is an absolute must.
The difference is dramatic, both in terms of ping time to the nearest router and in SSH interactivity. Yes, both priorize outbound and (to a certain extent) inbound traffic. But, believe it or not, the killer feature in Wondershaper is the outbound bandwidth limiter. This ensures that, even while running Direct Connect or BitTorrent servents, your Internet connection remains responsive.
And, best of all, both Wondershaper and Firestarter coexist peacefully!