Thanks, CP, it works!
Thanks, too, to Jon. I don't know ow it happened, but some time ago my version of Firefox suddenly appeared blotted out with news of an update. So I dutifully pressed the button only to be informed that the latest version was not compatible with my version of the Mac operating system. Try as I would, I could not get rid of of that wretched message on the screen. With the information Jon supplied I googled the version I required and was able to download it, and it works perfectly.
I'm very grateful to you both!
Hugh