Overall it looks good and useful, however:

1) It doesn't explain why some bookmarks give an error
2) It returns as "errors" perfectly working bookmarks.
E.g. https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/?src=search - It works fine but it's flagged as error
3) It should allow correcting multiple warnings/errors
4) Bookmarks paths are incomplete. This is very bad when trying to eliminate duplicates.

EDIT: that's it. After a couple of interactions it stopped working (I click check and nothing happens. Tried to close and reopen the tab, but it didn't solve. I'm going to uninstall it.