This manual is displaying in a Firefox tab, in the built-in PDF viewer (or Adobe Acrobat viewer)?

If you open a new blank tab using Ctrl+t, does it show up there?

If you click the Home button on the toolbar or press Alt+Home on the keyboard, does it show up as one of your home pages?

Thank you for your reply. No, it doesn't show up in either of those cases.

And when I closed the Firefox page there it was again!  :)

I'm puzzled that closing that window or tab isn't fully closing it. Can you use the jump list that appears above the Firefox button on the Windows taskbar to confirm that it's really closed when you close it, and not merely minimized?

Thanks again for your response. I think I have solved the problem. It seems I had inadvertantly 'pinned' the Manual. So when I managed to find and click the 'unpin' option, it no longer appeared. Regards, krisha