Simple Multiple Bookmarker — boiwantlearncode

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