I agree with what Stefan has said above Firebird is very robust on Windows but also on Posix systems and is used in large scale applications

Firebird on Large Big Iron servers : 512G-2TB of ram 100.000 users http://www.firebirdnews.org/?p=6797

Where firebird shine is embeeded mode also : full multithreaded and is easy to carry around with you

DBD::Firebird does have support for embedded mode Damyan created some of the test to run with embedded mode

The dabatabase that is to be used in LibreOffice Base is Firebird Embedded and the reasons are that Firebird is lot more powerfull than sqlite and a lot faster and easier to deploy/integrate than other databases https://wiki.documentfoundation.org/Development/Base/FirebirdSQL