To set the charset in Firebird must be in the database creation
CREATE DATABASE <database> 
  USER <username> 
  PASSWORD <password> 
  PAGE_SIZE <pagesize> 
  DEFAULT CHARACTER SET <charset>
See an example below:
CREATE DATABASE localhost:meter
  USER SYSDBA
  PASSWORD masterkey
  PAGE_SIZE 4096
  DEFAULT CHARACTER SET UTF8;
To change an existing charset 
ALTER CHARACTER SET charset SET DEFAULT COLLATION collation
Take the example:
alter character set utf8 set default collation unicode_ci_ai
If you use SET DEFAULT COLLATION on the default character set of the database, you have effectively defined the default grouping for the database.
If you use SET DEFAULT COLLATION in the connection character set, the string constants will be interpreted according to the new standard grouping. In most situations, this will make no difference, but comparisons may have another result if the grouping changes.