I don't understand what these bookmarks did.

Due to a change in Firefox 54. links and script calls to URLs that only have a protocol (e.g., http:, file:) are treated as invalid in websites. (URLs with no host name are now treated as invalid) I didn't realize it affected bookmarks.

Or it could be a coincidence and the problem might be something else.

The bookmarks don't do anything and of course they are not meant to be valid URLs. They are for my reference to split groups of URLs into something like a bullet-point list when I Show All Bookmarks and look at the bookmarks listed in that specific folder. Whereas before I could delete / edit / move these invalid bookmarks, as from now can't do anything with them.

If no one comes up with a solution, you might need to reinstall an earlier version of Firefox and edit each bookmark to a valid URL (you could use an internal address like about:blank or about:home as a placeholder). See: Install an older version of Firefox.

If that's too much manual labor, there may be a way using the SQLite Manager extension to do it with a database UPDATE query. However, that's not something I can recommend off the cuff without some serious testing.

Thank-you jscher2000. Much obliged for your help. I will keep this in mind. #Aces!