Do you mean the Bookmarks menu that drops from the classic top menu bar?

There was a bug that occurred after you use the Customize feature. It caused that problem in the window where you used Customize, but the menu still worked correctly in other windows (e.g., press Ctrl+n to launch a new window).

This was finally fixed in Firefox 55, but due to some serious bugs affecting bookmarks and history, Firefox 55 has only dribbled out to select users so far. If you can live with the workaround of launching a new window or using the context menu on the bookmarks menu that drops from the toolbar button (the one that looks like a clipboard, and is next to a star), that may be safer for the moment until you get updated to Firefox 55.

Please disregard my question. Apparently my problem was caused by having installed a Firefox update and not yet restarting Firefox. Restarting fixed that problem, but then my LastPass no longer worked. Had to go back to Firefox 54.0.1. I'm quickly losing interest in Firefox.

Just read your response. Thank you very much for your quick response. It was 55 that broke my LastPass.

There is a beta version of LastPass that should be compatible with Firefox 55. I'm not sure how soon it will become the official release, but you can get it here now if needed:

4.1.62a: https://addons.mozilla.org/firefox/addon/lastpass-password-manager/versions/beta

Thank you. I'll check it out.