I wanted to import my firefox passwords into a keepass2 .kdbx database. This guide was very helpful until the point where neither KeePassX2 Alpha nor MacPass have a way to import from XML (MacPass has the menu entry but as of this time it is not implemented)
From there, I downloaded the regular KeePassX (version 1.x) imported from the XML and saved as a normal .kdb file. MacPass fortunately handles .kdb files, and you can easily drag and drop groups from one MacPass window to another.
Thus the only thing left to do was to open the .kdb with the imported firefox passwords side by side with the .kdbx I wanted to get the passwords into, and drag the group from the left pane of the one window to the other.