Quite simple actually :)

1. Hit CTRL+SHIFT+B to open the Bookmarks Manager.
   
   
2. Click Bookmarks Menu in the left hand menu to get the list of folders in the right hand pane.
   
   
3. In the same right hand pane, click and hold down the left mouse button on the folder you want to move to the top of the list.
   

See screenshots. I moved the folder called Tennis to the top of the list.

Technically that Folder is going to be in either the Bookmarks Toolbar folder or the Bookmarks Menu folder - you can't move it above those folders in the Sidebar view.

Now, if you are trying to move a folder to the top of the Bookmarks Menu folder and it wants to go into the folder that is at the top already, instead of above that folder, settle for putting it below that folder and then move the top folder below the folder that you just moved. I've seen a glitch with trying to move a folder to the top position within a folder, and have assumed it was related to one of the Bookmarks extensions that I have installed - but maybe not. That "glitch" has been there since Firefox 3 was released for me.

If you are trying to move a folder or a bookmark out off the three main (root) bookmarks folders (Bookmarks Toolbar, Bookmarks Menu, Unsorted Bookmarks) then that is not possible.

• http://kb.mozillazine.org/Sorting_and_rearranging_bookmarks_-_Firefox

--- See also this extension:

• SortPlaces: https://addons.mozilla.org/firefox/addon/sortplaces/