Hi, it has been like that as long as I can remember. When you click on the star, the bookmark, by default, will go into Other Bookmarks - which you cannot change (prior to v48 known as Unsorted Bookmarks); however if you wish it to go to a different destination, keep your mouse pointer over the dialog box to prevent it from disappearing, then use the upper, downward pointing arrow on the far right, to select a destination > Done. (You can also click Other Bookmarks, then select a destination, but if you have a lot of folders you will also need to click Choose to see them). Bookmarks in Firefox.

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You can change the Default bookmarks folder using this extension. https://addons.mozilla.org/en-US/firefox/addon/default-bookmark-folder/