Final thoughts on this: firebird (fmrly interbase) did not achieve the level of success in the market that postgres, even though they may have been similarly positioned. My take: that disparity in success has more to do with postgres having a more open development model, stronger community, and (especially) timing; postgres was pretty well established in the open source world when Borland open sourced it around the year 2000. Firebird had (and has) some neat stuff, in particular a nice embedding option and strong windows support, but the market was already pretty crowded at that time.
Still around 2005 Firebird was more popular than Postgres (in the Czech Republic). I think after this time, the Firebird had not some wave of popularity (and Postgres did - RoR, PostGIS, ora2pg, json, ...), and it was too joined with windows desktop and Delphi. I have not any idea about current situation, and current performance of Firebird, but 15 years ago (Firebird 2.x), Postgres was better for web applications running on Linux server with more than thousands users. I did some benchmarks at 2005, and Firebird was really fast on Windows, and slower on Linux (slower than Postgres).
