My default browser is technically IE, and there's a good reason for this..
The Address bar, that's usually used for entering URLs, can be used to enter programs. Like you can type in 'charmap' and it loads up charmap, or 'cmd' to load the DOS emulation thing, or "C:" to pop up an explorer window at C:. I have an Address bar on my task bar for just this. But if I set my default browser to Firefox, it doesn't work (in fact, it tries to go to
www.charmap.com or something). Because IE integrates into windows, it can tell that when I type "F:" I want to access my F drive. (The downside to this is that I can't actually use the address bar on my task bar to enter URLs, I have to load Firefox and then type the URL).
I don't suppose anyone knows of a way around this, enabling Firefox to 'catch' programs and things like IE does. I'm sure that this is far too specific a thing for there to be any existing extensions for it..