BOOkmark Marker highlights all the links on a webpage that belong to your bookmarks. It provides a variety of options to customize the highlight. Many of the options are specific to a bookmark tag, so it’s possible to change the appearance of the link by changing tags of the bookmark. In addition it optionally displays the bookmark description, tags, and some other details in a popup that shows up when you mouse over a bookmarked link.
BooM will modify userChrome.css file under your Firefox profile directory to display the description field in Firefox's 'Bookmark This Page' window. The behavior is controlled by settings under General tab in BooM Options screen.