Roberto,

R> Has FireBird 1.5 some built-in function that allows me to act the same
R> way as with SQL Server ? (it should be a function that retrieves some
R> string value that I've setted in a Delphi program using a
R> TIB_Connection)

the closest you get with Fb 1.5 is by using SQL ROLES at login and
inspect CURRENT_ROLE in your triggers (mind you, it will evaluate to
'NONE' if no role was specified).
With Fb2 you can use the two system functions RDB$GET_CONTEXT and
RDB$SET_CONTEXT to set and get values as you wish.

Ciao
--
Nando Dessena
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