what happen to MYSQL database, is Firebird by default now?
Nothing. At least not recently. Firebird is the default database ever since MySQL changed their license and disallowed commercial software to distribute the MySQL installer bundled. (IIRC that was a few years ago when it was still MySQL AB, before Sun took over and way before Oracle bought Sun)
For a limited time you could get seperate installers from the Spacial homepage. One with and one without firebird included.
Nowadays the one without firebird is only available if you know where to find it.
However, that doesn't keep you from running SAM with MySQL. The only difference is the filesize.
While installing a database System (MySQL, MS SQL, Postgres or Firebird) before running the SAM installer is mandatory with the sambc-up.exe, you can (but don't need to) install sambc-fb.exe with Firebird without downloading and installing a seperate software by hand.