UPN login for external Webpage

I have a login for a PDU (power distribution unit) that pretty much requires firefox and that it needs to be run in an external windows, the embedded version has issues. The login has only username and password, however it is linked to our AD and uses either UPN (user@domain.com) or down level login names (domain\user) in the username.

I have my username and password stored in the credential repository but I don't see where in the configuration where I can specify either format or configure the format which is sent to the firefox plugin.

I have temporarily created a copy of my credential where I have domain\user as the username but I don't like to keep a 2nd copy of my credentials.

Is it possible to configure the output to send UPN for the username?

thanks