chromie
Hi, as you can see from my profile, I have a lot of experience with browser extensions.
I've read your specification and it's clear to me.
#4 is not possible, URL can't be masked in address bar, it always shows the actual location.
Regarding Firefox, I can do it too.
And for Safari, you'd need to obtain an license from Apple Store, I think it costs around $100, but I'm not sure that your extension would be approved by them, as it is security risk to users, redirecting them without their knowing.
Regards, Stefan