Any idea why?! And how to resolve?! It reverts to the undesirable resolution on every launch now, despite my 'fix' in Control Panel. The only relevant details I can add involve... 1) a Freeze which came from an undesirable tab that popped up in one of my three open windows, probably phishing or some such. 2) This was resolved by a PC Restore, followed by installation of some Mozilla-recommended security apps. Thank you!