As found in the LifeHacker article Four Experimental Firefox Extensions We’re Into.

Automatic Save Folder allows you to specify a place to automatically save files by doman and file name to certain folders.  I can never remember which download location is used by which computer, much less find my preferred location for each specific type of file.  It would be nice to not have to play hide-and-seek my downloaded files, or fish for the location that I really wanted them to go to in the first place.