Firefox has three top-level categories for bookmarks, all of which should be available in the sidebar: Toolbar, Menu, Other. You may be able to drag bookmarks between categories/folders in the sidebar, or you can use the Library window (Ctrl+Shift+O as in Organizer).

Normally Firefox will save new bookmarks in the last location where you saved a bookmark, so probably that changed and you need to change it back to your preferred category. To try to do that:

While viewing a page you bookmarked, click the gray star icon in the address bar to drop down the edit panel. Here you can change the category/folder where the bookmark is saved AND you can check the box at the bottom of the panel so that you always get this panel every time you save a new bookmark (to control what category/folder is used during the initial save).

Does that work?

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