I'd like to stick some instrumentation in some web pages and then if
my extension is loaded have the instrumented page call code in my

Right now I'm doing it as a Firebug extension and using xpcom to check
for my extension services. Is this the right approach? The xpcom code
is located in myext/components and a page can load the service even if
firebug and my extension haven't been loaded.

The extension aids in some very specific trouble shooting. If you
install it you'll have a need for it and understand the security
concerns involved.