How to convert integer, decimal, double or date to string?
This is very simple, since Firebird's built-in CAST operator does this very well:
select cast(22.5 as varchar(50)) from rdb$database;
select cast(integer_field as varchar(50)) from rdb$database;
select cast(date_field as varchar(50)) from rdb$database;
It uses default formatting: dot as decimal point, no thousand separator, dates in ISO-8601 format. Just make sure that size of VARCHAR field is large enough to hold the longest string representation of the datatype you are converting.
If you need special formatting, it's best to do it on the client side, in your application code. Short of that, you could combine string functions (SUBSTRING, CHAR_LENGTH, etc.) to extract the needed parts. There are also some UDF functions that can format the data for you. Take a look at rFunc and FreeAdHocUDF libraries for such functions.