After installing the add-on, I restarted FF 52.3.0 (running Mac OS X 10.11.6). Then I chose "broken bookmarks" and the add-on found 140.063 (yes 140 thousands!) bookmarks. Then I clicked on "check bookmark" and after 5 minutes I saw 211 checked but after 20 minutes the number of items checked was still 211. No numbers in warmings and errors fields. The add on is still working because I see in the middle of the window 3 circles moving ... and I cannot have any feedback nor choose any other option (like "duplicates", for instance). I'm (sadly) going to remove the add on ...unless I can have any suggestion to have the add on working ... I can't ever rate this add on...maybe too much bookmarks?