Start over. Export these to desktop then drag and drop into a tab and click on the ones want to keep and bookmark them again or Create a Folder in Bookmarks and drag and drop the ones do not want into the folder and delete once done.

I doubt you will find anything but you can have a look : https://addons.mozilla.org/firefox/search/?platform=mac&q=Bookmark%20Manager

You can manage the bookmarks in the Bookmarks Manager (Library; Show All Bookmarks).

• https://support.mozilla.org/en-US/kb/bookmarks-firefox

For "don't exist," there is an extension that can do a link check and alert you to those. For "don't use," I think you're on your own.

Warning: This is slow when you have a lot of bookmarks. Verrry slow.

https://addons.mozilla.org/firefox/addon/bookmarks-organizer/