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.