> What features of SQL Server am I going to miss if we use Firebird? Is
> there support for complex data types? Extended stored procedures?

Native XML support?

Few points from my experience.
My experience is OPTA driver for MSSQL. I dont know if there are
differences with MS jdbd driver.
http://www.inetsoftware.de/English/produkte/JDBC_Overview/ms.htm

* MSSQL has an excellent storedprocedure support for returning
ResultSet instances. SPs returning resultset and direct sql queries
behave exactly same at client side. And SPs does not need any weird
format to return scrollable rs.

* MSSQL7/2000 TSQL language for coding SPs could be a lot better.
Yokon (next MSSQL) finally has try-catch and SPs can use C# language
which I find a big plus.

* Yokon can directly publish SPs as a webservices, which might be a
usefull for integration. I dont know however, whether it requires IIS
frontend which is not a big plus.

* MSSQL can run a batch queries on the fly with full TSQL support. One
app we had to develop was ability to generate TSQL scripts based on
user parameters and then run it. Script could have 2-3 pages long and
each time had a different source code. Running such queries is a
performance problem in a thight loop, but ok for ocassional run or
scheduled tasks.

* Unicode/UTF8 support in MSSQL works as expected. If you need a
multinational data then you must evaluate this very closely in
Firebird. It has a fundametal problems in a current release. Maybe FB2
fixes problems.

* Firebird is a nice to embed with the application
* Firebird can be run on lowend machines with good performance
* Firebird does not "steal all your base" after installation
* Firebird2 should be a big impromevent once released.
* Firebird community is very active and helpful.

* If you use a common DAO pattern for data access, then you only need
a simple select/update/delete/insert syntax and thats what all dbs
give you. You dont need stored procedures etc... so MSSQL advantages
mean nothing here. This is where Firebird is a very valuable and
performance wise. (except UTF8 support which I always find a very
problematic).