The Free On-line Dictionary of Computing (30 December 2018):

stiffy
firmy

    (University of Lowell, Massachusetts) A
   3.5-inch microfloppy, so called because their jackets are
   more rigid than those of the 5.25-inch and the (obsolete)
   8-inch floppy disk.  Elsewhere this might be called a
   "firmy".

   [Jargon File]

   (1994-11-03)