Using internal and external http protocol handler in XUL app

Hi all,
I'm developing a stand-alone XUL app with a single window displaying a web application running on a local server. Somewhere in my main.xul file there is the browser pointing to the application:

<browser type="content-primary" id="webapp" src="http://url-to-webapp" flex="1"/>

No it happens that inside the webapp there are links to different http resources, which I would like to open in the (OS's) default web application. But when I set the preferences for my app to handle http externally (pref("network.protocol-handler.expose.http", true); and pref("network.protocol-handler.external.http", true);) the content of my <browser> element is opened externally, too (the same applies to the request for auto updates). So my <browser> elements keeps empty…

So, I need to be able to differentiate between "legal" internally handled http requests and requests that have to be handled externally. I would like to check a request before it's executed and decide to handle it over to the internal or external handler.

Any ideas?

Cheers and thanks, Daniel