Sorry for the late reply.
I see no reason why you cannot use FirePHP with your application.
I presume your client is written in JavaScript/HTML/CSS and can run in
Firefox?
You can initialize FirePHP in your PHP bootstrap file that calls
NuSOAP and then use the FirePOHP logging functions in your bootstrap
file as well as from within the NuSOAP library. The NuSOAP project may
be interested in adding FirePHP support for debugging to their
project. Maybe you can contact them to see what they think. You can
point them to this group and I will be available to answer questions
and help out with the integration if need be.
Hope this helps.
Christoph