> <parameter>
> <name>CharSet</name>
> <value>Cp1251</value>
> </parameter>

Parameter is called "lc_ctype" and values are ones that are in the
Language Reference. In your case that would be:

<parameter>
<name>lc_ctype</name>
<value>WIN1251</value>
</parameter>

Another option would be to include it in JDBC URL:

jdbc:firebirdsql:...mydb.gdb?lc_ctype=WIN1251

Best regards,
Roman Rokytskyy