Prior to Firefox Quantum versions, when you bookmark a page in Firefox, it will be saved to bookmarks menu by default, which no longer the case with current Firefox version after Quantum launch, the bookmark now will be saved to Other Bookmarks folder (formerly known as Unsorted Bookmarks).
While there is no option or hidden preference available to change this from happening, you can indeed stop Firefox from saving new bookmarks to the other bookmarks folder by using an add-on.
Stop Firefox from saving new bookmarks to ‘Other Bookmarks’ folder
Default Bookmark Folder
Default Bookmark Folder extension available for Firefox on AMO, allows you to choose default bookmark folder location — it could be bookmarks menu or Bookmarks toolbar or any other folder you’ve created- by clicking on the star icon in the toolbar or by selecting option in the context menu.
After installing the add-on, visit its options page, click on Firefox built-in bookmarking, and select the folder in which new bookmarks to be added.
The add-on also offers an option to add new bookmarks to top of the selected folder.
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