Currently midori doesnt import bookmarks from firefox. But there is a way to do this manually.
Export your bookmarks from firefox into html file. (Bookmarks → Manage bookmarks → Import/Export → Export into html)
Select Netscape as “input format” and XBEL as “output format”, select a file with your exported bookmarks and press “Convert”
Import file into midori. (Bookmarks → Import bookmarks)