Sorted! Thanks, MMario.

It does seem a bit bizarre though - I had to inform the computer about this totally fictitious modem, which it then imagined it had installed, and allopwed things to go forward, And then I uninstalled it. There seemed a clear assumption that telephone dial-ups are the only way to connect to the Internet - and I'd pretty well forgotten they were ever used.

And now I've got IE up there, I've no intention of using it either, except on occasion. (Though if only I could find a spellchecker for Firefox as good as HotLingo, which only works on IE...)