Although not directly answering your question re: false confessions in arson cases, it is well documented that one in four people who were wrongfully convicted but later exonerated by DNA evidence had falsely confessed to the crime. [
www.innocenceproject.org]
An interesting aspect of obtaining a false confession is that the interviewer is likely not aware that the confession is false. This American Life did a wonderful podcast on this subject where homicide detective Jim Trainum discusses this exact issue - obtaining a false confession without knowing it. If you have a few minutes to spare, it's worth the listen:
[
m.thisamericanlife.org]
Paul Bieber