https://addons.mozilla.org/en-US/firefox/addon/bookmark-topper/
By default firefox creates new bookmarks at the very bottom of their folders. The same happens when you drop a bookmark onto some folder or change the folder for some bookmark via the star dropdown interface in the address bar - it will become the last one in the folder.
This extension changes the behavior so newly created bookmarks would be placed at the top of their folders along with bookmarks dropped onto a folder.
There is a shortcoming though: if you drop a bookmark not onto a folder but exactly into the last position, the bookmark will still be moved to the top. So to place some bookmark at the very bottom, first place it last but one, next move the last one above it.
Important notice: bookmarks import order is reversed as well, so before importing make sure to temporarily disable the extension!