We had Interbase (which is Firedbirds predecessor) as our customer information and warehouse system. It's incredibly robust and (at least on Linux) quite fast.

We had Interbase (which is Firedbirds predecessor) as our customer information and warehouse system. It's incredibly robust and (at least on Linux) quite fast. We chose it because it already supported triggers when Mysql did not, allowing us to put the complete business logic into the db rather than into desktop applications. Today I'd prefer Mysql (now that it also supports triggers) because it's faster, you get better support and it's default for most frameworks.