I run FF 57.0.3 x64 under Win7 Pro x64. Finding empty folders and duplicated bookmarks works OK. Finding broken bookmarks appears to work until it hangs right at the end (363 total bookmarks, 360 scanned). It is still responsive, but it never finishes the scan. This add-on would fill a real need if it worked properly, but as it is I can't use it, so I've disabled it and will wait for improvements. I have other add-ons that deal nicely with empty folders and duplicated bookmarks.