I think not only app but from the very first firefox addon would be great too but I have different version. I think it will be very hard to push website owners to accept your btc tip method but as I see a lot of them have paypal donation buttons. With addon, it will be grest if users are able to pay 1$ or any amount with bitcoin and then you automatically donate it in usd but now as I realize my idea is hard and probably worse because of paypal tos. But addon would be great.