Why does Firefox move my newly added bookmarks?

Hello In FF31 I'm having the following problem: I've setup my bookmark menu with folders and separators. Now, when I create a new bookmark for a homepage (by dragging the URL into my bookmarks menu and dropping it where I want it) then the bookmark initially appears at the correct spot (where I dropped it - lets say for example between two already existing bookmarks or between a separator and a bookmark or wherever). However, as soon as I close the bookmark menu and open it again then FF has moved my newly created bookmark to the very bottom of the list. If I want to have the bookmark in the correct spot then I have to scroll to the very bottom of the list and drag the bookmark back to where I want it - after that it stays at the correct location. This bug (or feature?) is beyond frustrating... why on earth would I drag my new bookmark to a specific location within my bookmark menu if I wanted it at the very end of the list? Any ideas how to disable this feature? Or if it's a bug, any chance of a fix this side of Christmas?