I Found Something Rated 5 out of 5 stars
I Think This Will Do This...
It Adds right click options for bookmarks menu/toolbar bar (but not in library) to set icons for links that don't have any. Any links that have icons revert back to their original so not a full featured extension. Folders can be changed but revert back after restarting FF.
Download and Install It Off-Line .:
Based on This .: http://roachfiend.com/archives/2004/12/08/how-to-create-firefox-extensions/