The 4th paragraph in this article:
http://www.webtechniques.com/archives/2001/01/lilly/
says:

"... With few concurrent users, InterBase is fast on simple
reads and complex joins, but its performance drops sharply
under the stress of multiple queries and numerous concurrent
users. Because InterBase uses a nonshared architecture, as
user numbers increase, it must parcel data into ever-smaller
partitions, diminishing its performance levels. This major
flaw will likely be addressed if InterBase attracts skilled
open-source developers. However, the program is new to the
open-source world and still lacks the support of a strong
developer community."

How accurate is this statement? I've always tried to search
for a PostgreSQL vs. Firebird comparison and that is the
first I've found. So far the ff seem to be the case:

Footprint: Firebird is much(?) smaller than PostgreSQL
while providing a feature set that closely matches it.

Ease-of-setup/deployment:
Firebird is definitely easier to deploy under Windows
than PostgreSQL which requires futzing around with Cygwin
first; Under Linux, I have found Firebird to be quite easy
to set-up (although I doubt that would be the case for
a newbie who doesn't know how to read bash scripts or
setup rc files). Linux PostgreSQL setup could be even
simpler as it comes as a package with many distros.

Win/Linux/*nix interoperability:
As far as I can tell, Firebird wins this hands down.

SQL dialect:
Regarding SQL dialect, I find that MySQL has a friendlier
dialect and more polished/convenient built in user functions
than Firebird. However, the MySQL dialect is seriously crippled
in some critical aspects (lack of SQL features like subqueries,
triggers, views, SPs, etc...).

As I got used to it after a couple of days (not realizing that
you have to quote object names if they have lowercase
identifiers seriously tripped me up in the beginning), I found
the basic Firebird SQL dialect to be more or less OK.

I have no idea what the PostgreSQL dialect is like; anyone
care to add comments wrt that?

Admin tools:
Firebird's Windows-based tools should be more polished than
any Linux-based ones you can find (Quickdesk, minor bugs
notwithstanding is very nice) for any of the three OS
databases; For Linux based ones, it's probably a toss-up
with PostgreSQL maybe having a bit of an edge.

Features: PostgreSQL has Object-Relational, Firebird
doesn't. Aside from O-R, I get the impression that PostgreSQL
has a bit more extra advanced features, but the question is
are they worth the additional bloat (and how much bloat
do these actually add?).

Performance:
I would like to find out how accurate the statements in the
article are. Comments anyone?

I think a Firebird vs. PostgreSQL is as important as, if
not more so, than a Firebird vs. MySQL one. People who
are thinking of using PostgreSQL are likely to give Firebird
more serious consideration than MySQL.