This is odd, the option to 'sign in using browser' just came available and so I am now able to get into my portal that way. Waiting to hear back from my IT Security to see if they changed anything.
I did have windows updates apply during a restart yesterday afternoon (12/1421, ~ 3:00 pm EST). They may have just rolled back that change.
Well, the login dialog window from Pro 2.9 reappeared with username password form:
for reasons unknown. But I can say it had nothing to do with the favicon.ico not loading.