Quoted from a post I founded on another list:
The "seamonkey" package is available only on the "universe" repository. You'll have to enable it (if that hasn't been done yet)
If your question was "I want a version newer than the one available for my distribution", as as a rule of thumb I wouldn't advise you to do that unless you have a *really* good reason: you could mess up your system or fall into what is known as a "dependencies hell".
It would probably be better to upgrade your distribution itself, e.g. in your case upgrade to Kubuntu 8.10 if you need it.