Remove custom items from Firefox bookmark menu

If you want your Firefox bookmark menu to display only your own bookmarks without all those "Bookmark This Page" items, go to %AppData%/Mozilla/Firefox/Profiles/profilename/chrome and edit userChrome.css. If it's not there, just create it. Now paste this:

#menu_bookmarkThisPage,
#subscribeToPageMenuitem,
#subscribeToPageMenupopup,
#menu_bookmarkAllTabs,
#bookmarksShowAll, #organizeBookmarksSeparator,
#bookmarksToolbarFolderMenu,
#bookmarksToolbarFolderMenu+menuseparator,
#bookmarksMenuPopup menuseparator[builder="end"],
#menu_unsortedBookmarks { display: none !important; }

#BMB_viewBookmarksToolbar, #BMB_viewBookmarksToolbar+menuseparator,
#BMB_bookmarksShowAll, #BMB_bookmarksShowAll+menuseparator,
#BMB_bookmarkThisPage,
#BMB_subscribeToPageMenuitem,
#BMB_subscribeToPageMenupopup, #BMB_subscribeToPageMenupopup+menuseparator,
#BMB_bookmarksToolbar, #BMB_bookmarksToolbar+menuseparator,
#BMB_bookmarksPopup menuseparator[builder="end"],
#BMB_unsortedBookmarks { display: none !important; }

#appmenu_showAllBookmarks, #appmenu_showAllBookmarks+menuseparator,
#appmenu_bookmarkThisPage,
#appmenu_subscribeToPage,
#appmenu_subscribeToPageMenu,
#appmenu_subscribeToPageMenupopup, #appmenu_subscribeToPageMenupopup+menuseparator,
#appmenu_bookmarksPopup menuseparator[builder="end"],
#appmenu_unsortedBookmarks { display: none !important; }

The last part (in orange) will hide default items in Firefox appmenu bookmarks as well, so leave it out if you want to customize bookmark menu in  a toolbar only.