I was eventually able to access the website using an old Internet Explorer browser, but I still can't use it with Firefox. Very odd, since Firefox has been my default browser for years and the website worked fine with it as recently as yesterday.

There's been no change in or update to Firefox & I didn't even log out of my computer last night - just put it in hibernation. So nothing has changed - except with the Hyatt website, which now won't let me log in.