it has a simple solution, go to the bookshelf symbol just beside the star(bookmark symbol)--> click the shelf symbol---> then click on search all bookmarks---> a new window will open --> press delete to whichever bookmark u wish to delete.

Solution2 Press Ctrl+Shift+B and press delete to remove bookmarks

Have a look at these articles with screenshots, they will help you understand what you should do.

• Use bookmark folders to organize your bookmarks
• Bookmarks in Firefox

Note you may drag and drop bookmarks within the bookmarks library. You may wish to put the bookmarks from the bookmarks toolbar in another folder instead of just deleting them.

If you are removing them because you have run out of space and need to scroll consider using folders or nested folders on the bookmarks toolbar.