You can only do so from the Bookmark Library window. Just press Ctrl + Shift + B to open that window. Then you can right click on the bookmark you want to copy and select copy. Then you can paste it into the appropriate folder. If you want to copy multiple bookmarks, hold the Ctrl key and click each bookmark you want to copy.

You can't create a duplicate bookmark simply from the star on the Firefox address bar. Firefox is designed to try and prevent accidental duplicate bookmarks, so it won't let you click the star multiple times to create multiple bookmarks.

For more information on bookmarking websites in Firefox, refer to the Bookmarks in Firefox documentation.

Hope this helps.

Thanks so much for the help! I just finished sorting the last of my bookmarks yesterday, thanks to the information you provided, and I'm very happy with the new configuration. No more seemingly endless scrolling to put something into place on my bookmarks list!

Wesley Branton said

You can only do so from the Bookmark Library window. ...

You can save duplicate bookmarks by using the "drag'n'drop" method, too.

Click-hold and drag the Tab into the Bookmarks Toolbar or into the Bookmarks Sidebar (if you use it); or even to the Bookmarks Menu item if you have the Menu Bar showing all the time, although opening the folders from that menu drop-down is quite a bit twitchy - overshoot just a smidgeon and the drop-down will close.