As another review says, this addOn seems unfinished. My particular complaint is as follows. On one run, a bookmark will be identified as broken. On another run, another bookmark will be identified as broken. Both bookmarks actually work fine. I infer that the addOn is not waiting long enough for sites to respond.

EDIT: I seem unable to respond to the developer's reply to my comment. So I will reply here. The AddOn seems to work better now but I continued to find at least one site that responded very slowly, but did respond, and was (mis?)flagged as not responding by the Organizer. That site: https://www.gutenberg.org/