Tabelas e views:

SELECT RDB$RELATION_NAME FROM RDB$RELATIONS;

Somente tabelas:

SELECT RDB$RELATION_NAME FROM RDB$RELATIONS
WHERE RDB$VIEW_BLR IS NULL;

Somente views:

SELECT RDB$RELATION_NAME FROM RDB$RELATIONS
WHERE NOT RDB$VIEW_BLR IS NULL;

Observação:

Para não incluir as tabelas e views de sistema, acrescente o filtro (RDB$SYSTEM_FLAG = 0 OR RDB$SYSTEM_FLAG IS NULL) na cláusula WHERE. Exemplo:

SELECT RDB$RELATION_NAME FROM RDB$RELATIONS
WHERE RDB$VIEW_BLR IS NULL AND (RDB$SYSTEM_FLAG = 0 OR RDB$SYSTEM_FLAG IS NULL);