press ctrl+shift+B. then click bookmarks menu. select the folder you want to rename, and in the lower bar, you can change the name of the folder. (see screenshot)

If you have a problem then it's probably because you are running an old version of FF. I recommend installing the current version Here

If you mean changing names of the default folders then you can do that via the right-click context menu (Properties) in the Bookmarks sidebar (Ctrl+B).