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Multiple Bookmarker

Hold command or ctrl and left-click to manually select multiple tabs and bookmark them under the same selected folder.
Hold shift and left-click to select multiple tabs in a row with a single click.
Bookmark URLs that already exist in a folder will not be duplicated.

This add-on is open-source and can be found here:
https://github.com/boiwantlearncode/multiple-bookmarker

Install
Find add-on button at the side. This opens a list of your bookmark folders. Multi-select tabs you want to bookmark using command/ctrl or shift and left-click and click the folder you wish to place them under. The result.