Could you double-check your add-ons and make sure you don't have one called Default Bookmark Folder? Here's how. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

Anything here that relates to bookmarks which might be affecting how they are saved? Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

Any improvement?

Oh, one other thought:

You will see all new bookmarks listed under "Recent Bookmarks" across all your folders. To check what folder one of those is saved in, you can right-click it and select "Show in folder" to scroll the Bookmarks Sidebar to its location.

The "Edit This Bookmark" dialog that opens if you click the highlighted star has a checkbox with "Show editor when saving" (shortcut key in the dialog is Alt+S). This item is at the bottom just above "Remove Bookmark" and "Done". You can click the highlighted star a second time to open the "Edit This Bookmark" dialog.

See "How do I change the name or location of a bookmark?"

• https://support.mozilla.org/en-US/kb/bookmarks-firefox