Nigel & Robert,

Do a search on Dalton Calford and give a look on some threads he wrote
about firebird scalability. He works with really big databases
(30GB+) in Firebird and with 250+ users (there are posts form
02-08-05).

Here's something
http://www.firebirdsql.com/index.php?op=useful&id=calford_1
(Helen moped up Dalton's writings and made it, IIRC)

HTH
Andrew