This is a Windows problem
This URI problem is a result of the OS being unable to handle/sanitize requests properly. Letting any application (bugs and all) register itself as an arbitrary URI handler is kind of wacky at best. It's doubly dangerous because it happens without notifying the user.
Please make it clear that this "Firefox problem" doesn't happen on any OS other than MS Windows. Then we can correctly judge if the application or the OS is to blame.