Now when you do queries, the plugin will ask firebird library to provide strings in unicode. For that the database must have a character set defined. But what happens if none is defined? In that case the try to convert the guessed character set to Unicode will destroy probably the umlauts in the text. The solution can be to cast the encoding like this: CAST(FIRMA as varchar(100) CHARACTER SET ISO8859_1)
So for each field we select, we redefine the datatype with a cast operation and fix the encoding. On the long run the database encoding should be defined to fix this and have the plugin automatically provide correct strings.
PS: Same works in FileMaker with our MBS FIleMaker Plugin and the SQL functions.