I think it should work as an option where you want to see the tabs (if at all): top/bottom/left/right/nowhere. In that case, there is no need to turn off the original one. This is how Vivaldi does it:
In the "View" menu, where there already is a "Sidebar" and "Toolbars", "Tab bar" would be perfect. If the user turns it off, they can turn it back here.
I have the same problem after logging in. After submitting my email address in the login form, I got an email with the subject "Welcome to Mozilla Connect" and a link in it, which leads to a 401 page like this.
