>>Is there a way to know what a trigger or a stored procedure is doing?

http://www.firebirdfaq.org/faq214/ How to debug a stored procedure.

>>I'll try to explain it better: in a language like Delphi, C, C++, etc., I
>>can put a message box inside a procedure or function, so I can know the
>>value of a variable or the line of code which is being executed.

IBExpert (Firebird database admin tool) allow you to debug stored procedure in the same way that you can with Delphi. Even the shortcuts (F7, F8. etc) are the same.

It has breakpoint and watches.

I can highly recommend it.