Unfortunately you can not do that it has been asked like forever and always gets pushed aside. You will have to export your bookmarks and strip out what you do not want being careful to maintain the integrity of the exported bookmarks backup file. Then import the bookmarks. into Firefox. The imported bookmarks will get added to a folder at the end of your "Bookmarks Menu" bookmarks folder. Move them to where you want them and then delete the folder you added to the "Bookmarks Menu". If you automatically sort your bookmarks (automatic sorting is a bad idea), be sure to turn it off beforehand.