We got the idea from The Firebird Book, and after that I can't find mention of it being deprecated in 2.0 release notes or later. From the 2004 book, page 745 (I know it's older than my daughter ;-):
Version 1.5 forward
Supply a list of one or more directories, separated by semicolons (;), under which sort files may be stored. Each item may include an optional size argument. in bytes, to limit its storage.....
TempDirectories = userdata\sortfiles 5000000
And also in the 1.5.6 release notes https://firebirdsql.org/rlsnotesh/config-fb-conf.html, and then hanging around in something that claims to be the Firebird 2 Administrators manual, but now I look closer it isn't obviously a core fb site: http://www.janus-software.com/fbmanual/manual.php?book=admin&topic=42
I can't find it mentioned anywhere else though, except in things like http://tracker.firebirdsql.org/browse/CORE-2151 where the author was still using it...
I didn't know it had been deprecated, I just thought that as an optional feature most documentation didn't mention it.
I'll find an alternative plan....