First, please update to Firefox 19. Then, to rename a bookmark, simply Press Control + B, right click the bookmark, properties, then rename.

I upgraded to 19,

• press ctrl+B hold both down & 'r' click on the folder - I get add a bookmark
• press ctrl+B release them both & 'r' click on the folder - i get a small window, top element is "open All Tabs"

I never see a window with properties. Henri

Are you trying to rename a bookmark or a folder?

I would like to be able to rename either. I tried both of the suggestions on both a folder, and a bookmark within a folder, and the results are the same. Now I am in the Library, and the library HAS the focus, when I try this. Perhaps I'm in the wrong place. Henri

OK, It has to be done outside of the Library. The ctrl+B brings up the bookmarks, then thr 'R' click gets the properties....

You can change the Properties (including the name) of a Bookmark or Tag in the Details (Properties) pane at the bottom of the right pane in the Bookmarks Manager (Bookmarks > Show All Bookmarks: Shift+Ctrl+B) after selecting an item.
To change the name of a folder you need to open the parent folder via the left pane to make that folder appear in the right pane, so you can select it and see the name in the detail pane.
You can make changes to a Bookmark in the Bookmarks menu drop down list or in the sidebar (View > Sidebar > Bookmarks, Ctrl+B) via the Properties entry in the right-click context menu.
You can open a bookmark in a tab and click the yellow (blue on Mac) star in the location bar to open the "Edit This Bookmark" dialog and see in which folder a bookmark is located.