True Universal Open Source Database
Firebird is a truly performant database management system. It scales impressively from a single-user model to enterprise-wide deployments with multiple databases running with hundreds of simultaneous clients.
See Who Is Using Firebird SQL?
Firebird SQL versus Microsoft SQL Server
Bigger is Better
Firebird (FB) max DB size is unlimited, max table is 32TB. Where else Microsoft SQL Server Compact Embedded Database (better know as SQL Server Express) max DB size is 10GB (4GB is v2008). 
Hidden Cost Is Never Good
SQL Server Express is free, but beyond Express is $3717 USD starting price.  Further increasing the cost is SQL Server requires Windows server starting $882 USD  + every 5 users need Client Access License $219 USD. 
Freedom Is Good. End of Support Is Bad.
Firebird open source freedom is guaranteed.  SQL Server Express can continue, discontinue, file size reduced or feature restricted depends on Microsoft interest. Microsoft has discontinued or end support on many products. Example 1, Windows Server 2012 foundation (refer to page 1) Example 2, Windows XP (7).