When adding a new bookmark by clicking the star icon, the "Add Bookmark" popup appears, in the "Location" selectbox, the last used bookmarks folder is selected by default, to choose another bookmarks folder, we need to click the Location selectbox then we click "choose" and the bookmarks folders tree expands, then we select a desired folder.
I suggest that , instead of the normal Location selectbox, firefox uses a searchable scroll-able selectbox to search the bookmarks folders tree (and search the full path of bookmarks : folder1/folder2/folder3/ )
Similiar already posted idea:
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