Alvin,

> 1) Did there a feature at firebird to control on concurrent user log
> into database?

What do you mean by this?

> 2) Any utility (freeware) to convert interbase 5.6 to firebird 2.0?

Right, you want someone else to do all the hard work for you
and you want him to provide you with a tool, for free?!

Best thing you can try is to create a backup with IB5.6 and
restore it in Fb 2, dialect 1.

Good luck.

Martijn Tonies
Database Workbench - development tool for Firebird and more!
Upscene Productions
http://www.upscene.com
My thoughts:
http://blog.upscene.com/martijn/
Database development questions? Check the forum!
http://www.databasedevelopmentforum.com