As already written to quite some comments, the normal deletion speed is several thousands bookmarks per second. A considerable slower speed is almost certainly due to a bad configuration, e.g. another extension which plugs into the bookmark API and takes time for every bookmark deletion. You might want to retry with a plain firefox configuration (after saving and restoring only your bookmarks). BTW, the code for the chrome and firefox extension of Bookmark Dupes is practically identical as you can see on github.