I have a long time experience using Firebird, more than 10 years,
and I can say that it is very stable even on Windows XP. It is
also robust, easy to install and use, there is a good DBD module
for it (DBD::Firebird) and there is also a good administration tool named Flamerobin.
I use it with small databases, but I see that is also your intent so I have no reason not to recommend it.
Regards Stefan.
Update: This doesn't mean it can't handle large databases, it just means that I don't have experience with large databases.