I've managed to solve it.
I've enabled it via OLS's php.ini file, editing it as it was in the Apache's file. Via webconsole, I haven't find it how to configure.
I have a Debian server where there are Apache2 and OpenLiteSpeed webservers installed. After installing php-interbase, I can see the Interbase module available at Apache2 (via phpinfo), but not in OpenLiteSpeed. How do I activate it in OpenLiteSpeed?