This is a total UI nit, and has been around for a while; punt it out as much as you want.
On /signin if you enter an secondary email address that is not known to browserid.org, the UI transitions to show "firstname.lastname@example.org is not registered. Would you like to sign up instead?", and provides two different links to move to /signup. The UI nit is that the 'Sign In' button is still visibly clickable (it actually takes no action, or at least, does not make any /wsapi call when clicked)
The same behaviour is seen for /signup, if you swap 'known' for 'not known' and 'Verify email' for 'Sign In', etc.
Closing in favour of GH-2120.
OK, Verifying as a "duplicate" of #2120 which is already closed.