hello paulcr, please follow this guide in order to customize the user interface in firefox: Customize Firefox controls, buttons and toolbars

there are two similar looking bookmark buttons - one opens the bookmark sidebar, the other one the bookmarks drop-down menu...

Thanks Philipp. I see now that when the original "button" disappeared I replaced it with the slightly different looking button. However the "original" is not shown as an option when I customise the toolbar. Additionally I note the "original" is supposedly part of the default toolbar, but when I do a restore of the default toolbar it doesn't return. All very odd, and I don't even know how it disappeared in the first place.

Problem solved but it seems to me the toolbar was not behaving as I would have expected. Trial/error did the trick.

You only see the Bookmarks Menu button if the Menu bar with the Bookmarks menu is hidden (View > Toolbars or Firefox > Options).
If the Bookmarks Toolbar is visible then the Bookmarks Menu button is displayed on the Bookmarks Toolbar as part of the Bookmarks Toolbar Items (bookmarks), but you can move it from the right side to the left side of the Bookmarks toolbar if the Customize window is open.
Otherwise the Bookmarks Menu button will appear on the right hand side of the Navigation Toolbar.