When bookmarking multiple tabs at once, firefox forces the user to save the bookmarks into a new folder, then the bookmarks should be moved to the desired folder in the "Bookmarks Manager".
It would be efficient if Firefox give possibility to bookmark multiple tabs at once, to an existing folder while keeping option to creating a new one.
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You should already be able to do this by selecting multiple tabs, then dragging them as a group into an already existing folder.
Let us know if that works for you... or if you're looking for something else.