@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

/* regular bookmarks menu */
menu.bookmark-item, menuitem.bookmark-item { min-width: 200px !important; max-width: 500px !important; } 

/* bookmarks toolbar folder dropdown menu */
#bookmarks-ptf toolbarbutton menupopup menuitem { min-width: 100px !important; max-width: 500px !important }