What do you mean with 'cannot select a bookmark separator' ?

A separator is meant for improving readability/accessibility (i.e viewing purpose) and you wouldn't have to select it.

Can't you add a bookmark separator via the right-click context menu or remove the separator or is there another problem ?

I have bookmark separators in the wrong places. In order to delete one I first need to to select it. I know that to do this I right click on it. But when I do nothing happens. it remains unselected.

Try clicking on it. Then right-click.

Easiest to deal with separators is in the Bookmarks Manager (Library) and in the sidebar as you can select this item. If you try to do this on the Bookmarks Toolbar or in a drop-down menu then you need to position quite precise (i.e. between the highlight of the item before and after the separator).