A nice little app Rated 4 out of 5 stars

It does exactly what it says in on the tin - goes through your bookmarks folders and finds all dead links or other errors, then gives you a list so you can check that it's got it right before you delete them. It found something like 60 errors in my 500 bookmarks going back 15 years, and only one of them was a false flag. Note that this has to be run from the Add-Ons toolbar, which might confuse you if you don't have it activated.

This review is for a previous version of the add-on (1.2).