Somewhere buried inside the screen full of HTML is probably the error message. Try to copy the whole thing and paste it into a text editor and see if you can find the error message.
Or it may be a "404 Page Not Found", which means some files weren't installed properly on your server or the permissions on some files are set wrong and they can't be read.

Thanks for the suggestion jordanlev, but the dialog is modal and the text is niether fully shown nor able to be copied. I went ahead and deleted the page and created a new page type with the defaults I wanted, namely Exclude from Nav, Exclude from Page List, and Exclude from Search Index (all checked), but new pages created from that page type have the attributes, but not checked! And of course when I check them and save, same error dialog.

That was it! Rather, that was not it. Setting the properties worked fine in FF.

Cheers