Please don't spread FUD about any open source products in an open source community. Firebird is not that BAD, in fact it's very far from bad. Which version of Firebird have you tried? Have you tried the latest version (v.1.5.4 and v.2.0.1)?
I use both Firebird and PostgreSQL in production environment. Both are FAST for common operations. For some specific operations, each has its own advantages and disadvantages. That's why I use both.
Since you (beli0135) said Firebird is bad, now I will be on Firebird side to stop this FUD. Firebird is fast, has small footprint, officially supports embedded mode, easy installation and configuration, almost zero maintenance, etc. But it does perform pretty slow on very huge hits especially when involving very complex queries. I don't count a left join as a complex query. I have a Firebird (v.1.5.3) database with hundreds of tables and over 6 million records on a particular table, runs very stable with number of hits over 100 users (daily use).
PostgreSQL is also a good product and performs as good as Firebird. It does performs better (and more stable) on very huge hits involving complex queries, compare to Firebird. But the installation is not as easy as Firebird, the configuration is more complex, no support for embedded mode, and requires slightly higher CPU and memory.
So, I think it's just a matter of requirement. For easy maintenance with low to medium hits, I prefer Firebird. But for a high load transaction, I prefer PostgreSQL since it performs better than Firebird. In fact, I have more Firebird databases compare to PostgreSQL databases on my production zone.