Which Linux distribution is recommended for Firebird?
Short answer: the one you feel most comfortable with
Linux distributions are merely a collection of Linux kernel, supporting GNU tools and various applications on top. The thing that really differs them is the package system and custom configuration utilities. None of those two affect Firebird performance. So, you should really use the one whose package management and configuration tools you find most suitable. Many users have reported success in production with Red Hat Enterprise Linux, Suse Linux, Mandrake and Mandriva, Ubuntu Server, Debian, Slackware, CentOS, Gentoo.
In any case you should try to stay away from desktop or beta/testing distributions like Fedora or Ubuntu and use server versions: RHEL/CentOS and Ubuntu Server. While Firebird might run flawlessly, those systems aren't stable/optimized to be server systems. Here's an example of a real-world case: