Rated 2 out of 5 stars
I see no way to remove the icons. I've uninstalled the add-on and removed the bookmark, restarted firefox and add the bookmark again and it creates the favicon again. I would expect that icons created by this add-on will be removed when you do the above. I got tired of the fake icons that didn't really mean enough to me so I've removed it.This review is for a previous version of the add-on (0.3).
Work in progress
This feature is on my todo list. The problem is that Firefox API does not trivially support this. Anyway, your problem is that favicons are not only stored in the bookmarks but also in the BROWSING HISTORY. So in order to completely discard identfavicons, first delete the affected bookmarks, then wipe the browsing history (from "clear private data"), and then recreate the bookmark. (I think browser restart is not necessary.)