<<I have an old FB1.5 database working from 2005 until today. It has a little more than 700 MB. Works on Windows 7 Professional...
Finally, I decided to update to new server etc. Simply, I thought - gbak backup on FB1.5 and restore to FB3...>>

Please read

Upgrading from Firebird 1.5 to 2.5 and beyond (.pdf) (161.31K)
by Pavel Cisar

http://www.ibphoenix.com/files/conf2016/upgrading-Cisar-2016.pdf

It will help...

Regards
Paul