The Jargon File (version 4.4.7, 29 Dec 2003):

firefighting
 n.

    1. What sysadmins have to do to correct sudden operational problems. An
    opposite of hacking. ?Been hacking your new newsreader?? ?No, a power
    glitch hosed the network and I spent the whole afternoon fighting fires.?

    2. The act of throwing lots of manpower and late nights at a project, esp.
    to get it out before deadline. See also gang bang, Mongolian Hordes
    technique; however, the term firefighting connotes that the effort is
    going into chasing bugs rather than adding features.


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

firefighting

   1. What sysadmins have to do to correct sudden operational
   problems.  An opposite of hacking.  "Been hacking your new
   newsreader?"  "No, a power glitch hosed the network and I
   spent the whole afternoon fighting fires."

   2. The act of throwing lots of manpower and late nights at a
   project, especially to get it out before deadline.  See also
   gang bang, Mongolian Hordes technique; however, the term
   "firefighting" connotes that the effort is going into chasing
   bugs rather than adding features.

   (1994-12-01)