> I was wondering where I could get a list of aggregate functions supported in Firebird 1.5.

In the documentation (e.g. IB6-Beta documentations from www.ibphoenix.com).
FB1.5 does not implement any new aggregate functions.

> In particular, I'm interested in STDEV. Thanks

Only COUNT(), MIN(), MAX(), SUM(), AVG()

Ivan
http://www.volny.cz/iprenosil/interbase/