Hello, if you read the add-on description page https://addons.mozilla.org/en-US/firefox/addon/bookmark-search-plus-2/, you will see it has to fetch the favicon for each bookmark to be able to display them (the little images at front of bookmarks). This is because Firefox doesn't give access to them. The add-on does that only once.
Alerts mean that you have plenty of bookmarks to malwared URLs .. :-)

You can deactivate favicon fetching in the options page, and you won't get any alert. But then all icons will be the standard "no favicon" icon. We are just waiting that Firefox gives us that access -> https://bugzilla.mozilla.org/show_bug.cgi?id=1315616

Thank you, aaFn.