Bookmark toolbar dropdown menu does not always display bookmarks
I have a problem with the bookmarks toolbar. If I display a dropdown menu and then move the mouse pointer over the menu entries, the next level of entries for a folder are not displayed. This doesn't apply to all the folders, some display the next level, some do not. This problem is getting progressively worse. All the entries are present on 'Bookmark Management' I use the latest version of Firefox on the latest version of Ubuntu.
I sync the bookmarks between several devices, the toolbar works properly on the other devices. (Ubuntu, Mac OS, Android)