Please try :

SET TERM !! ;

alter PROCEDURE SP_VERSION_NOS(
    PORTION smallint,
    SEPARATOR char,
    EARLIER_TO integer)
RETURNS (
    VERSION_NO varchar(80)
)
AS
declare variable i smallint;
declare variable SEPARATOR_POS Integer;
declare variable VERSION_PORTION varchar(80);
declare variable VERSION_PORTION_INT integer;
BEGIN
    FOR SELECT
        a.VERSION_NO FROM VERSION_NOS a INTO
         :VERSION_NO
    DO BEGIN
        i = 1;
        while (:PORTION > :i) do begin
            select position(:SEPARATOR in :VERSION_NO) from RDB$DATABASE into :SEPARATOR_POS;
            VERSION_NO = substring(:VERSION_NO from (:SEPARATOR_POS + 1) for char_length(VERSION_NO));
            i = 1 + :i;
        end
        select position(:SEPARATOR in :VERSION_NO) from RDB$DATABASE into :SEPARATOR_POS;
        i = 1 + :i;
        VERSION_NO = substring(:VERSION_NO from 1 for :SEPARATOR_POS - 1);
       
        VERSION_PORTION_INT = cast(:VERSION_NO as integer);
        if(:EARLIER_TO >= :VERSION_PORTION_INT) then begin
            SUSPEND;
        end
    END
   
    /* SELECT p.VERSION_NO FROM SP_VERSION_NOS ('3', '.', '456') p; */
END!!

SET TERM ; !!