After installing Firebird 1.5 on Ubuntu Dapper I ran into a strang problem. I first installed firebird2-classic-server and everything worked.
Then I installed firebird2-dev and suddenly I couldn't connect anymore to my database, not even locally as root.
If the fixes in the article above doesn't help you, make sure that the /usr/lib/firebird2/run symlink points to /var/run/firebird2, and that the directory actually exists!
mkdir /var/run/firebird2 as root did it for me.
The big question now is: Why did this directory disappear in the first place?