Firebird 1.5.2 (embedded)

I have small (VB) app which currently uses MySQL and where I am making
changes which have to all succeed or not. I'm newbie to Firebird.

The VB app uses SQL to achieve all this in step like
dbconnection.execSQL("START TRANSACTION")
dbconnection.execSQL("some update")
dbconnection.execSQL("more updates")
dbconnection.execSQL("COMMIT")

I want to maintain app so I can use MYSQL and Firebird depending on
clients choice, but use of START TRANSACTION (which I thought was
standard SQL) returns error (Dynamic SQL error SQL error code -104
unknown token START)

What I want is SQL to achieve same in Firebird, eg do I need to set
autocommit off in some way instead of START TRANSACTION and then just
do COMMIT or what?

Any help/pointers appreciated

Tony