If you have found the bookmark via a search, but do not know what folder it is in then you can open this bookmark in a tab. You can open the bookmark in Offline mode (File > Work Offline) to prevent opening the link from internet. You can click the blue highlighted star to open the "Edit This Bookmark" dialog. This doorhanger shows details (properties) including in what folder the bookmark is located. You can open the full folder tree via the Down arrow button that is positioned outside the folder field and scroll down (drag the scroll thumb or use the mouse scroll wheel) until you see the selected folder.