Most of You Will Want to Use a Different Add-on Rated 4 out of 5 stars
THIS ADD-ON CAN ONLY DELETE BOOKMARKS ONE AT A TIME.
IF YOU WHO HAVE LOTS OF BOOKMARKS:
USE "BOOKMARKS CHECKER - CHECK FOR BAD LINKS" INSTEAD
'The Bookmarks Checker - check for bad links' extension can scan for duplicates as well as bad links. It will automatically select every single duplicate for you and allow you to delete all (say, 6000) bookmarks in one click.
Maybe this one could use a multi-select function. It does have one advantage over the other extension, though: this extension displays them along with their location (specifically, it sorts them by URL, whereas the other sorts them by location). If you have OCD, like me, you'll *want* to keep the bookmarks you might have stashed in folders, neatly organized, so use this one... on the other hand, if you don't really care, use the bookmarks checker.
P.S. please don't be too impatient with the developer. It's hard work with no pay.