It seems to work great but then I started looking more closely at some of the URLS that were flagged as broken and several of them are fine. At first there was a problem with sites being flagged that pop up the secure site warning and ask for permission to access the http site, so I enabled, "Don't enable HTTPS only mode" in the Firefox settings. However, now I still get perfectly fine links in the list. I noticed some of them have the pop up to accept or edit cookies so does this add-on falsely report sites with this as broken? This add-on is not much good if you still have to manually go through the list of "broken" links it finds to check if they are actually broken.

helpful but could use a better UI. For example, ability to hide results or ignore certain bookmarks or entire domains (ie., google.com/*)

Waste of time. Wish I had taken the previous 1 and 2 star ratings to heart.

It would be great to have a single button that will remove any additional URL links on every folder without having to go to each one individually and select it. Automation is key.

It does an excellent job in checking for broken and invalid links.

However, the detection and selection of duplicate bookmarks falls very short.

It often gives false positives where it detects a bookmark as a duplicate if starts with the same url but is actually a different url. This is deadly and a huge bug imo.

There's really no way to SELECT ALL the duplicate bookmarks with one button. You have to select each folder and so on. Not efficient.

Finally, there's no instructions and developer hasn't updated the add-on since October. Looks like he's abandoned it.

Confusing to use and even when I thought I worked it out I ended up with more duplicates than what I had to start with!

Part of this app works well. With over 3000 bookmarks, this app stops checking for broken URLs at about 2400. Stops cold. The pause doesn't work at this point. There is no way to "select all" and no way to delete the bookmarks with broken URLs.
I uninstalled the app, reinstalled the app and got the same problem again. The duplicates part of the app, including the merge, works well. The broken apps? Not so much.
The search for an app goes on.