That's a very weird question. I guess, there might be browser specific stuff, but of course we'd have to know, what browser you're using. Talking about that, if it's firefox, then theoretically you could edit the code yourself, as it's freely available. Practically, it'd probably be an awful amount of work.
The only real solution I can think of, is if you have your internet connection running through some kind of router, which you can work with. There are some places around, where you get free WiFi here, and you normally get one site, whatever you enter into the address field, on which you have to accept their conditions or something like that. Not quite sure how they do it (haven't really thought about it either), but I'm sure one could find out.
But I guess the big question is: WHY would you want to do something like that?
Greetings,
Nepomuk