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.