It is possible to right click in the toolbar and add a separator.

I don't want to add a separator. One solution would be to delete one of the separators, but I cannot do that. Seems that particular pair are each fixed. I cannot add a separator in the gap any more than I can add a bookmark.

There are three main Bookmarks folders and bookmarks and separators can only be created in one of those folders and its sub folders.

• The "Bookmarks Toolbar" folder stores the bookmarks that you see on the Bookmarks Toolbar (Firefox > Options or View > Toolbars).
• The "Bookmarks Menu" folder stores bookmarks that show in the Bookmarks menu and are created via "Bookmarks > Bookmark This Page" or via "Bookmark This Link" in the right-click context menu.
• The "Unsorted Bookmarks" folder stores bookmarks that are created by clicking the star at the right end of the location bar.

If you want the new bookmark or folder to appear on the Bookmarks Toolbar then create that new bookmark or folder in the "Bookmarks Toolbar" folder. Otherwise create the new bookmark or folder in the "Bookmarks Menu" folder or in a sub folder in that folder. If you click the star on the location bar then a new bookmark is created by default in the "Unsorted Bookmarks" folder. You can click the star another time to open the "Edit This Bookmark" dialog and move the bookmark to another bookmarks folder. If you use "Bookmark This Page" (Ctrl+D) then the bookmark is created in the "Bookmarks Menu" folder.

How far apart in time were these different events? I ask because:

Do you want to roll back your bookmarks to an earlier date when the menu appeared the way you want? Unfortunately, you would lose any other additions, edits or changes you've made in the meantime. If that's worth it to you, check out this article on restoring one of the automatic backups: Restore bookmarks from backup or move them to another computer.

I had no hard drive, and gather the bookmarks file was not something I thought to back up, because I have not found it with recovered files. I made the mistake of removing the last of the default bookmarks between the separators after I'd re-entered enough bookmarks that I don't want to go back. It's actually no problem. The pair of separators just takes up a bit more space than one. Thanks for the suggestion.

Thanks cor-el for your specificity. It does not intersect with my problem. I re-established my bookmarks toolbar without problem. I have several folders and subfolders in the bookmarks menu, with their respective bookmarks (mostly moved from Unsorted Bookmarks as I generate these). When you click the Show Your Bookmarks button in the upper right, the dropdown has six sections. Below the section containing "Bookmarks Toolbar" and "Unsorted bookmarks" with arrows to their right, and above the bookmarks menu section there is a section that I emptied. I cannot make the space go away be deleting a separator; I cannot make us of the space by putting anything (back) there. (I've forgotten what was there.)

The "Show your bookmarks" drop don list shows some default entries (menu items) that you can't remove and other items that are in the Bookmarks Menu folder. You can check if you have a separator directly at the top of the Bookmarks Menu folder by selecting this folder in the left pane in the Bookmarks Manager (Library).